Re-exploration Programs for Petroleum-Rich Sags in Rift Basins

By Xianzheng Zhao, Fengming Jin, Lihong Zhou and 2 more

Re-exploration Programs for Petroleum-Rich Sags in Rift Basins covers the geological characteristics and potential of oil-rich depressions in a rifted basin. It describes up-to-date research and technology, detailing the current status of exploration. The overall aim of the book is to guide a new round of hydrocarbon exploration of petroleum-rich depressions, contributing to breakthroughs in re-exploration and a substantial increase in reserves. Chapters discuss the reservoir forming theory of oil-rich depressions, characters of hydrocarbon migration and accumulation in a weak structure slope, key elements of reservoir forming of deep buried hills and inner curtains, and more.

Other topics covered include complex subtle reservoir recognition techniques, deep layer and buried hill high speed drill technology, recognition of buried hill reservoir and hydrocarbon, high efficiency enhanced oil recovery, and finally, methods of secondary exploration of oil-rich depressions and the development of a workflow to guide research and exploration.

• Provides up-to date knowledge and expertise on the geological characteristics and potential of oil-rich depressions in a rifted basin
• Based on a decade of experience, program deployment, and geological theory on continental basin exploration
• Gives practical guidance for exploiting green and brown fields
• Helps the reader understand how to increase reserves and production
• Ideal as a guidebook for sustainable large-scale exploration and exploitation of a continental rifted basin

• Language: English
• Publisher: Elsevier Science
• Release date: Aug 15, 2018
• ISBN: 9780128161548

Xianzheng Zhao

Xianzheng Zhao is President of PetroChina Dagang Oilfield Company, and a well-recognised expert designated by the Hebei Provincial Government. He is an adjunct professor at the China University of Petroleum in Beijing, China University of Petroleum in East China, and Yangtze University. He obtained his masters and PhD degrees from the China University of Petroleum in Beijing, and was awarded the title of Professorial Engineer in 2005. Zhao is a member of AAPG, SEG, SPE, the Geological Society of China, and is co-chair of the Petroleum System and Reservoir Panel of the Petroleum Committee of the Chinese Petroleum Society. He is deputy editor of China Petroleum Exploration, and on the editorial board of Petroleum Exploration and Development and Marine Oil and Gas Geology. He has published 15 monographs, and co-authored 131 papers. He has devoted himself to the study of oil and gas exploration and development in faulted depressions, weakly formed structural slopes, subtle buried hills and coal bed methane in continental rift basins.

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provinces.

Preface

Continental rift basins are developed in the Eastern China. Through extensive petroleum exploration practices since the 1960s, some theories and cognitions (e.g., terrestrial facies of petroleum and composite hydrocarbon accumulation) have been formed. These theories and cognitions play an important role in guiding and promoting petroleum exploration in continental rift basins, thereby contributing greatly to the petroleum industry in China.

As a typical continental petroliferous rift basin, the Bohai Bay Basin contributes 25% of China’s total oil resources and 40% of China’s total proved OIIP. It is one of the major oil- and gas-producing basins in China, with its production in 2013 making up 25% of China’s total. Oil-rich sags are the major contributor to petroleum reserves and production within the Bohai Bay Basin, with proved OIP and annual oil production as 82% and 73% of the basin, respectively. Currently, the resources in these sags have been highly proved and explored, but there is still abundant remaining petroleum. Moreover, the petroleum exploration in these sags is greatly imbalanced. For purpose of the state energy strategy of keeping stability in the Eastern China, it is vital to continuously deepen the exploration in oil-rich sags in order to achieve stable growth of oil and gas reserves.

Objectively, the structural plays in these oil-rich sags have been highly explored, where it is difficult to find massive reserves. Since the beginning of the 21st century, a lot of studies have been conducted for oil-rich sags, especially on the hydrocarbon accumulation conditions and enrichment patterns in stratigraphic-lithologic plays, with the support of alternative exploration strategy. Some new geological theories and cognitions were successively put forward, including sag-wide oil-bearing for hydrocarbon-rich sags, sub-sag oil accumulation for faulted sags, and faulted slope controlling sand, composite transport, facies-potential controlling accumulation for subtle (stratigraphic-lithologic) plays. Meanwhile, some innovative exploration engineering technologies were developed, such as sag-wide merged 3D seismic survey, facies-controlled precise reservoir prediction, and complex reservoir stimulation. By applying these new theories and technologies into the new round of exploration in the oil-rich sags, multiple hydrocarbon accumulation models have been established, contributing to the shift of exploration targets from positive second-order structural zones to structural flanks, troughs and synclines and from massive reservoirs at the top of buried-hill and Cenozoic structural plays to subtle buried-hill plays and stratigraphic-lithologic plays. Thus, the petroleum exploration domain and space are greatly expanded. Through practices, a new thinking of re-exploration for oil-rich sags is proposed. It points out that the re-exploration for oil-rich sags represents an overall innovation and application of geological theories, exploration methods, and engineering technologies and a systematic undertaking to realize massive reserve additions.

On the basis of practical exploration in the Jizhong Depression, Huanghua Depression and the Erlian Basin over the past decade, this book presents the necessity of re-exploration for oil-rich sags and provides the connotation, conditions and methods of re-exploration. This book elaborates the techniques, including sag-wide structure reconstruction, sedimentary reconstruction, and reservoir reconstruction, precise source rock evaluation and dynamic resource simulation, and relevant research results. This book comprehensively analyzes the hydrocarbon accumulation models and re-exploration practices in different zones/belts and prospects within oil-rich sags such as Raoyang, Baxian and Qikou. This book also summarizes key engineering technologies for re-exploration, such as sag-wide merged 3D seismic survey, reservoir prediction by seismic sedimentology, and complex reservoir stimulation. With substantial content and elaborated data, this book can help to supplement and deepen the petroleum exploration in oil-rich sags and provide a reference to the exploration in similar petroliferous basins or oil-rich sags.

This book was compiled under the leadership of Xianzheng Zhao and Fengming Jin, who determined the general structure and outline. Chapters therein were completed separately by relevant professionals after multiple rounds of discussion. The Preface and Chapter 1 were written by Xianzheng Zhao, Quan Wang, Weipeng Yan and Xin Li; Chapter 2 by Fengming Jin, Chuanzhang Tang, Shenghui Yuan and Yuansheng Chen et al.; Chapter 3 by Hongwei Zhang and Fuqing Tian et al.; Chapter 4 by Jianping Wu and Zhouqi Cui et al.; Chapter 5 by Chunyuan Han, Yiming Zhang, Yongjun Guo and Xiongying Dong et al.; Chapter 6 by Qiang Luo, Xuefeng Ma, Jian Wang, and Yulei Shi; Chapter 7 by Xianzheng Zhao, Fengming Jin, Quan Wang, Bingda Fan and Peiyu Xie et al.; Chapter 8 by Fengqi Qing and Menghua Wang et al.; Chapter 9 by Fengming Jin, Fuxiang Zhao, Bo Cai, Hua Shen, Mingshun Zhou, Qingfeng Meng and Anying Luo et al.; Chapter 10 by Dexiang Yang, Jingwang Liu, Fengxiang Hou, Weining Dan and Xiwei Li et al. The full text was finally checked and verified by Xianzheng Zhao and Fengming Jin. Finally, many thanks to Professor W. John Lee and Academician Zhijun Jin, who provided many valuable suggestions for the book's content, and offered help and support for its successful completion.

Chapter 1

Connotation and Workflow of Re-exploration

Abstract

The secondary exploration method centering on overall recognition, overall evaluation, and overall deployment has been proposed, mainly including building the merged 3D seismic data platform of the whole sag, on this basis, reconstructing the structure, deposition and reservoir, etc., evaluating the distribution of oil and gas resources in detail, working out the oil and gas reservoir accumulation model of multiple domains, and formulating an integrated reserve enhancement and productivity construction plan.

Keywords

Secondary exploration; Basic geology reconstruction; Distribution of oil and gas resources; Reservoir forming model; Overall exploration

Chapter Outline

1Necessity of Re-exploration for Oil-Rich Sags

2Connotation of Re-exploration for Oil-Rich Sags

2.1Definition of Re-exploration

2.2Connotation of Re-exploration

3Workflow of Re-exploration

3.1Constructing the Sag-Wide Merged 3D Seismic Data Platform

3.2Reconstructing the Basic Geology of the Sag

3.3Quantitatively Characterizing the Spatial Distribution of Oil and Gas Resources

3.4Creating the New Model of Multiprospect Hydrocarbon Accumulation

3.5Multiprospect Overall Preexploration

3.6Integration of Reserve Addition and Productivity Construction

1 Necessity of Re-exploration for Oil-Rich Sags

In China, continental basins, especially continental rift basins, are widely distributed. The Bohai Bay Basin is one of the important petroliferous basins in Eastern China and is endowed with excellent hydrocarbon accumulation conditions in oil-rich sags. Extensive petroleum exploration has been carried out in this basin for over half a century since Well Hua 8, drilled on the Xinzhen structure in the Dongying Sag, produced 8.1 t/d oil from the Paleogene on April 15, 1961. By the end of 2013, the basin had a cumulatively proved petroleum geological reserve of 140.52 × 10⁸ tons, as 40% of China’s total production and contributed an annual production of oil and gas equivalent up to 7596 × 10⁴ tons in 2013, as 25% of China’s total production, playing an important role in China’s petroleum industry.

According to the classification of Yuan and Qiao (2002), by resource abundance > 20 × 10⁴ ton/km² and oil resources > 3 × 10⁸ tons, the oil-rich sags of the Bohai Bay Basin are major contributors of reserves and production, and also the main accumulators of remaining oil. Many scholars and explorers have been working on ways to deepen exploration into the oil-rich sags of the Bohai Bay Basin. Jia et al. (2004, 2007, 2008) proposed the key technologies for exploring stratigraphic-lithologic plays, including seismic reservoir prediction and sequence stratigraphy. Zhou Haimin et al. proposed six precise exploration techniques, including precise secondary 3D seismic survey, precise oilfield geology study, and precise selection of drilling method (Zhou et al., 2003; Liu et al., 2005). Zhang (2004) deemed it critical, for seeking oil in old blocks, by using approaches beyond traditional ones. Meng (2005) discussed the deepening of exploration from the perspective of thinking innovation, higher seismic data quality, and development of leading technologies. Cai et al. (2007) and Chen et al. (2002) investigated the re-exploration opportunities in old plays and old oilfields in case the enrichment patterns in these plays/oilfields were further clarified. Xu (2012) proposed a new approach for offshore re-exploration in large and medium oil/gas fields, through overall research to acquire innovative geology cognitions and reduce the subsurface uncertainty, by way of combining optimal technologies and fully utilizing available data. Deng (2005) systematically analyzed the effect of exploration in the Bohai Bay Basin from five aspects, which played an important role in guiding future offshore exploration.

Some Chinese scholars addressed re-exploration from the perspective of exploration theory, technology, and practice, but no one has dealt with the connotation of exploration and supporting theory/technology with regard to system engineering. After analyzing the exploration degree, resource potential, and prospectivity of the oil-rich sag in the Bohai Bay Basin, we think that it is necessary to execute a new round of overall exploration for oil-rich sags with abundant remaining oil and gas, called re-exploration, with the support of new geological theories and new engineering technologies.

Petroleum exploration has been carried out for more than 100 years in the lower 48 states of the United States, and it has experienced the initiating stage, rapid growth stage, stable growth stage, and decline stage. Up to 2009, 76.7% of recoverable oil quantities had been demonstrated, and the discovery maturity is 76.7%. Compared to the exploration history of the United States, the Bohai Bay Basin lies in the stable growth stage; thus there is great potential and opportunity to deepen the exploration (Fig. 1.1).

Fig. 1.1 Exploration stages of the Bohai Bay Basin and the United States. (A) Proved oil reserves and proved percentage in the lower 48 states of the United States. (B) Proved oil reserves and proved percentage in the Bohai Bay Basin.

According to the petroleum resource status of the Bohai Bay Basin, the oil-rich sags demonstrate a highly proven percentage of hydrocarbons, but nevertheless have great potential for remaining resources. There are 14 oil-rich sags in the onshore and shallow offshore of the basin, including the Raoyang, Baxian, Langgu, Qikou, Nanpu, Damintun, Liaohe West, Liaohe East, and Dongying Sags, where an additional petroleum geological reserve of 50 × 10⁸ tons is expected to be proved. Therefore, these sags are still significant for reserve additions and production growth in the future.

The oil-rich sags of the Bohai Bay Basin are in the stage with a highly proved percentage and a high exploration degree. However, the exploration degree is apparently different depending on plays, exploration intervals, and structural trends. Specifically, it is high for structural plays but low for stratigraphic-lithologic plays, high for positive structure zones but low for sub-sag belts and negative zones, high for shallow-middle formations but low for deep-ultra-deep formations, high for massive structural plays at the top of buried hill but low for intraburied-hill and buried-hill slope plays, high for lacustrine clastic sandstone plays but low for (subaqueous fan) conglomerate, carbonate, and volcanic rock plays, and high for conventional plays but low for tight plays. The plays, exploration intervals, and structural trends with a low exploration degree may become new exploration and alternative exploration targets once they are further investigated and explored (Zhao et al., 2016a, b, c, d).

Along with extending exploration to more prospects and types, further understanding is obtained on hydrocarbon accumulation conditions and enrichment rules in continental (rift) basins, which can help to guide and promote the further exploration in oil-rich sags. For example, the sag-wide oil-bearing theory is conducive to multiplying the exploration coverage, which extends, from only second-order structure zones, to the entire basin dominated by petroleum-rich sags or the entire sag, dominated by petroleum-rich sub-sags (Zhao et al., 2004). Accordingly, based on the brand new concept of lithologic-stratigraphic plays, the exploration is carried out at the lower slope-trough in the oil-rich sags. In the northern Songliao Basin, the additional proven oil reserves are 10 × 10⁸ tons in the lower slope of oil-rich sags (e.g., Gulong) in the exploration province, which effectively supports the stable production at 4000 × 10⁴ tons for the Daqing Oilfield Company. In the Jiyang Depression, guided by the new theory of faulted slope controlling sand, composite transport, facies-potential controlling accumulation, large-scale reserves have been discovered in subtle plays, recording an efficient exploration with additional proven oil-in-place reserves of 1 × 10⁸ tons for 30 consecutive years.

In the Jizhong Depression, the research on hydrocarbon accumulation and enrichment rules in major oil-generating sub-sags of oil-rich sags gave rise to the new theory of sub-sag oil accumulation in rift basins (Zhao et al., 2009, 2011). This theory is manifested in several aspects. First, sand distribution is controlled by multiple factors. Distribution of sedimentary sand bodies in the sub-sag belt is controlled by multiple factors, such as bounding fault pattern, structure zone type, systems tract, slope-break belt, and sedimentary micro-facies. Second, hydrocarbon accumulation conditions are favorable. Traps in the sub-sag belt have the advantages of early development, early hydrocarbon charge, and good preservation conditions. Third, oil enrichment in the sub-sag belt is controlled by the hydrocarbon generation intensity threshold, the principal confluence pathway, and the critical reservoir scale. Fourth, hydrocarbons in the sub-sag belt are distributed commonly and complementarily in structural plays and stratigraphic-lithologic plays. Fifth, hydrocarbon accumulation models are diverse. Good hydrocarbon accumulation conditions in the sub-sag belt are conducive to forming diverse stratigraphic-lithologic accumulations (Fig. 1.2). This theory boosts the exploration value of sub-sags and lays a theoretical foundation for overall evaluation and sag-wide exploration in the rift basins. Guided by this theory, several zones with large-scale reserves have been discovered in the Jizhong Depression, including the Nanmazhuang West sub-sag and Hejian sub-sag of the Raoyang Sag, the Liuquan sub-sag of the Langgu sag, and the central sub-sag of the Shulu Sag.

Fig. 1.2 Hydrocarbon accumulation modes in sub-sags.

Moreover, in order to intensify the exploration in oil-rich sags, great endeavors are being made for a (second-round) 3D seismic survey and precise large-area merged 3D data processing, and the large-area merged 3D seismic data volume gradually covers the (main part of) sag. For example, a 3D seismic data volume with an area of up to 10,000 km² covering the Langgu, Baxian, and Raoyang Sags in the Jizhong Depression has been established, providing a substantial basis for the new round of overall research into the oil-rich sags.

2 Connotation of Re-exploration for Oil-Rich Sags

2.1 Definition of Re-exploration

Re-exploration refers to the process in which a new round of overall cognition, overall evaluation, and overall deployment is executed for oil-rich sags of the rift basins, according to new geological theories and with the support of new engineering technologies and new exploration methods, in order to realize great reserve additions in multiple aspects (Zhao et al., 2016c).

Re-exploration is applicable under three preconditions: (1) an oil-rich sag still contains abundant remaining resources; (2) a sag-wide large-area merged 3D seismic survey is realized; and (3) existing geologic understanding or exploration techniques are no longer sufficient for making new significant breakthroughs and progress.

For oil-rich sags, re-exploration is very different from exploration in aspects of seismic data, hydrocarbon accumulation models, major exploration targets, exploration technologies, and exploration methods (Table 1.1). In the Bohai Bay Basin, exploration is conducted mainly for structural plays in positive second-order structure zones through sag identification and play selection, by using 2D and local 3D seismic data and guided by the composite hydrocarbon accumulation pattern and the theory of young source and old reservoir buried-hill hydrocarbon accumulation. In contrast, re-exploration is a new round of overall exploration characterized by overall cognition, overall evaluation, and overall deployment with the support of applicable and advanced engineering technologies (including drilling, logging, and reservoir stimulation), targeting all plays (dominantly lithologic-stratigraphic plays) in the entire oil-rich sag, on the basis of the sag-wide 3D seismic data volume, and according to the new theories like sag-wide oil-bearing for petroleum-rich sags and sub-sag oil accumulation for oil-rich sags (Zhao et al., 2012).

Table 1.1

2.2 Connotation of Re-exploration

(1)Re-exploration is so-called for sags where exploration has been completed. It is not a simple extension and repetition of the preceding exploration that targets the structural plays in positive second-order structure zones, but a new round of overall exploration with more systematic deployment, broader prospecting, and more diverse targets under the guidance of renewed hydrocarbon accumulation theory and exploration technologies. It should not be constrained by the exploration results but be conducted systematically and comprehensively assuming the sags are new exploration provinces. It is not a repetition of the previous work but employs existing or new achievements to guide or adjust the exploration deployment.

(2)Re-exploration mainly targets oil-rich sags. After years of exploration, the oil-rich sags become dominant in exploration and development. With highly proved percentages of oil and gas, but still with abundant remaining resources, the oil-rich sags remain critical targets for seeking large-scale profitable reserves.

(3)Re-exploration is based upon the sag-wide or large-area merged 3D seismic data. On the one hand, over years of operations, the 3D seismic survey has covered the entire or most parts of the oil-rich sag, and the 3D seismic data platform for such a sag or its majority part can be set up through merged processing by using new technologies. On the other hand, rapid advancement of seismic technologies has enabled a significant quality improvement of seismic data newly acquired and reprocessed. Thus, the stereoscopic structural and sequence stratigraphic framework, covering the entire or most parts of the sag, is established to facilitate the cognition of structural and sedimentary characteristics, providing a valuable and essential basis for re-exploration.

(4)Re-exploration should take the renewed cognition and overall evaluation as the principle. An exploration in a new zone, where the geological data were absent, was usually begun with obtaining data and parameters and then extended gradually after the breakthrough point was determined. As to re-exploration for oil-rich sag, which is assumed as a new unit but with much data and many discoveries, cognition, evaluation, and deployment are all new arrangements on the basis of previous working results, targeting all possible accumulations in all geologic bodies within the whole entity of a sag. Re-exploration highlights the overall strategy in all aspects.

(5)Re-exploration is a sag-wide full-azimuth project targeting multiple layers and multiple prospects. In the Bohai Bay Basin, for example, early exploration was mainly concentrated on the shallow-medium layers in slope belts and positive structure zones, where a lot of structural and paleo-buried-hill accumulations were discovered, and the theory of composite hydrocarbon accumulation was proposed. Since the beginning of the 21st century, significant breakthroughs and large discoveries have been made successively in the exploration for stratigraphic-lithologic, low buried-hill and intraburied-hill plays. The sub-sag belt is no longer a forbidden zone; meanwhile, abundant cognitions on the characteristics and rules of hydrocarbon accumulation in stratigraphic-lithologic plays have promoted the terrestrial oil-generation theory. Therefore, re-exploration should be sag-wide (or basin-wide) and fully azimuthal, targeting multiple layers and prospects, without the forbidden zones in terms of planar/longitudinal distribution and reservoir types.

(6)Re-exploration should inherit the practice of precise exploration. Geologists from the Jidong Oilfield Company put forward the concept of precise exploration for the Nanpu Sag, involving the precise second-round 3D seismic survey, precise regional geological study, precise oilfield geological study, precise selection of exploration well-drilling model, precise research and development of logging interpretation technology, and precise organization and site management. According to this concept, the Jidong Oilfield Company achieved a great reserve addition and annual production increase in the old onshore exploration province with the coverage of only 570 km². Surely, this practice of precise exploration should be adopted and followed in re-exploration. Only in this precise way, can the mature areas be consistently and thoroughly explored.

3 Workflow of Re-exploration

As mentioned earlier, re-exploration is not a simple extension and repetition of the preceding exploration, but rather a new round of sag-wide overall exploration targeting multiple layers and multiple prospects. It is designed to find new large-scale reserves and improve the resource conversion rate in oil-rich sags through innovating the thinking, transforming the perspective, renewing the method and updating the means.

Re-exploration for oil-rich sags places the emphasis more on the combination and integration of high-resolution sequence stratigraphic division, precise source rock evaluation, hydrocarbon generation and accumulation simulation, new hydrocarbon accumulation models, and other data, factors, and methods, on the basis of merged 3D seismic data volume. In combination with exploration practice, the re-exploration method characterized by overall cognition, overall evaluation and overall deployment is developed (Fig. 1.3). It includes the reconstruction of the basic geology of sag, a precise quantitative characterization of resource distribution, the establishment of a multiprospect hydrocarbon accumulation model, overall multiprospect preexploration, and integration of reserve addition and production increase. The re-exploration method is an extended and improved exploration that is centered on multifactor analysis (including source rock, reservoir, caprock, trap, migration, and preservation).

Fig. 1.3 Workflow of re-exploration for oil-rich sags in the rift basins.

3.1 Constructing the Sag-Wide Merged 3D Seismic Data Platform

Previous exploration in oil-rich sags usually targeted structural plays in the positive second-order structure zones. Thus, a 3D seismic survey was deployed in these zones, and a single block of a 3D seismic survey covered 50–200 km². However, no 3D seismic survey was carried out in the sub-sag belts, negative structure zones, and flanks of positive structures. Meanwhile, survey times and techniques varied from block to block, resulting in very different data quality and thus making the data unqualified for merged processing and further identification of stratigraphic-lithologic traps. Therefore, it is essential for re-exploration to construct the sag-wide merged 3D seismic data platform.

First, the data quality and exploration potential in the blocks, with 3D seismic data acquired, are systematically assessed. A second-round 3D seismic survey is conducted for the block(s) with high exploration potential but poor seismic data quality. Second, a high-resolution primary 3D seismic survey is conducted for the data-blank blocks with exploration potential. Third, the coordinate system is uniformly defined and the pins are normalized for the entire exploration province; meanwhile, key technologies including establishment of the near-surface model, multiprospect precise prestack denoising, merged high-precision velocity modeling, amplitude, phase and frequency consistency, and time-difference correction are applied to realize the merged processing of data from different blocks of the sag and finally form the sag-wide merged 3D data volume platform.

At present, a sag-wide merged 3D seismic data volume has been established covering the Raoyang, Baxian, and Langgu Sags in the Jizhong Depression. It lays a good data foundation for the establishment of a high-resolution sequence stratigraphic framework, precise structural study, and precise sedimentary sand depiction. It also provides an important support and safeguard for the overall research of oil-rich sags and the identification of stratigraphic-lithologic traps, ultra-deep buried-hill traps, intraburied-hill traps, and other types of traps.

3.2 Reconstructing the Basic Geology of the Sag

On the basis of the sag-wide merged 3D seismic data volume, the precise structure interpretation, sequence stratigraphic analysis, seismic-sedimentary facies analysis, reservoir diagenesis evolution analysis, and other technologies are applied to identify the characteristics of structure, sedimentation, and reservoirs in the sag. Accordingly, the basic geology of the sag is reconstructed to provide an important basis for deepening the comprehensive study.

3.2.1 Structural reconstruction

The previous structural study for oil-rich sags was based on the merging of 2D seismic data and block-specific 3D seismic data. Along with the deepening exploration and the initiation of precise exploration, some prominent problems have occurred, such as inconsistent stratigraphic division for play fairways and blocks, unclear conversion relation of belts, and unsystematic study for characteristics of structural horizons, which impede the accurate understanding of the structural characteristics of the entire sag.

As to the structural reconstruction for re-exploration, the sag-wide isochronous stratigraphic framework is established after sag-wide well-to-seismic stratigraphic correlation, according to the program developed using the well-to-seismic data of main lines and through fine horizon calibration for critical wells, unifying the geologic zonation based on sag-wide well-to-seismic correlation, and setting up the merged 3D drilling zonation databases, with the support of the sag-wide merged prestack time 3D seismic data platform. Then, the sag-wide regional framework interpretation and precise structure interpretation for target areas are carried out. Finally, variable-velocity mapping is completed by using the sag-wide velocity model to provide the sag-wide high-precision industrial map. With these achievements, the oil-rich sag can be divided into structural zones for research into structural characteristics and eventually the structure units with exploration potential can be confirmed.

3.2.2 Sedimentary reconstruction

A continental rift basin presents strong tectonic movement and separation, with variable strata and sediments laterally, which leads to a multiplicity of stratigraphic correlations. In this circumstance, it was impossible to form a uniform sequence division criteria and scheme for the entire sag previously, when a block/play was deemed as the study unit, resulting in incomplete sequence grading and difficult sequence correlation, and directly restraining the depositional system research and industrial mapping under a unified sequence stratigraphic framework. However, the sag-wide 3D seismic data volume platform can help solve these problems. Guided by the sequence stratigraphy, modern sedimentology, and exploration theory for stratigraphic-lithologic plays, the sequence stratigraphic analysis, three-phase analysis and modeling, sand identification, and reservoir prediction methods are used to deepen the precise sequence stratigraphic division and correlation, sequence development, and controlling factor analysis, and identify the longitudinal and lateral distribution of depositional systems constrained by the third-order and fourth-order sequences. Thus, sedimentary mapping is completed by a precise way based on a systems tract, instead of the previously extensive method based on formations/members, so that the lake basin area (including deep-source rock) is clarified and the distribution of favorable sand bodies in layers is determined.

3.2.3 Reservoir reconstruction

As the oil-rich sags are increasingly explored, the deep clastic reservoirs as well as deep buried-hill and intra-buried-hill carbonate reservoirs in the sub-sag belts and intraslope belts, which were rarely dealt with previously due to limited data and techniques, appear gradually to become the primary exploration targets. Effective components of these reservoirs are vital for the exploration success. Therefore, the characteristics of these reservoirs and the main controlling factors for high-quality reservoirs should be identified in order to predict the ideal plays for a new round of exploration.

According to the research on the accumulation and preservation capacity of major oil-bearing formations (e.g., Wumishan, Gaoyuzhuang, and Cambrian) in deep buried-hill reservoirs, the original texture and pores of deep buried-hill carbonate reservoirs mostly experienced recrystallization, dolomitization, and other secondary reworking processes such as karstification, which gave rise to the accumulation space comprising fractures, cavities, and connected pores. The reservoirs can be classified as micro-fracture-pore, peusdo-pore, fracture-cavity-pore, and dissolved cavity-fracture types. Meanwhile, the relationship between the porosity, permeability, and burial depth of the buried-hill carbonate reservoir shows that the reservoir properties are less influenced by the burial depth. The existence of reservoirs with good properties is also possible in deep buried hills. This cognition provides an important basis for deep buried-hill exploration.

The research into the deep clastic reservoir performance in the Raoyang and Baxian Sags shows that the deep Sha 3 Member, Sha 4 Member, and Kongdian Formation mainly contain broad sand bodies of braided delta front facies. The reservoir rocks are dominated by feldspathic sandstones, with the reservoir pore space mainly consisting of secondary pores, including dissolved intergranular pores, intragranular pores, intercrystalline pores, and mold pores. According to the analysis of the diagenesis as well as its intensity, antigenic minerals, and pore evolution, the diagenetic facies are divided into four types, i.e., mechanical compaction diagenetic facies, early cementation diagenetic facies, strong corrosion-weak cementation diagenetic facies, and later-cementing alternation diagenetic facies. The strong corrosion-weak cementation diagenetic facies belts control the type and distribution of secondary pores and determine the development of a mid-deep reservoir space, contributing to the effective reservoir zone. Influenced by the barrier layers such as thick mudstone, the mid-deep reservoirs are generally under abnormally high pressure, making these reservoirs feature good physical properties. Due to high-quality source rocks in the deep formations, the early petroleum generation and early charge restrain the cementation, to effectively preserve the secondary pores. Therefore, the deep clastics can develop the sweet spots with good properties, which ultimately evolve into large-scale hydrocarbon accumulations. The relationship between the physical properties and hydrocarbon potential of mid-deep (> 3500 m) clastic reservoirs in the Raoyang-Baxian Sag shows that the lower limits of porosity and permeability of deep effective reservoirs are 8% and 0.3 mD, respectively, and the lower limit of and burial depth of effective reservoirs is up to 4200 m, thus expanding the effective exploration space.

3.3 Quantitatively Characterizing the Spatial Distribution of Oil and Gas Resources

Over years of exploration activities, there are still abundant remaining oil and gas resources in the petroleum-rich sags. Previously, such resources in the sags, plays, and layers were mostly predicted by basin modeling and scale analog methods. The results obtained only represented the total quantities of hydrocarbon accumulated; on this basis, it was difficult to confirm the favorable accumulation zones or units and select optimal exploration plays or targets. The exploration research and practices show that the traditional genetic method can foresee the oil and gas resource distribution but cannot remove the human factor, because the resource volume depends on the accumulation coefficient and analog coefficient; the conventional statistic and analog methods are relatively objective to the discovered reservoir type and the resource volume, but cannot be used to dynamically describe the accumulation process and directly predict the reservoir position and prospective zones.

Accordingly, the new round of resource appraisal mainly adopts the advanced Trinity reservoir-forming modeling system, which can quantitatively characterize the dynamic reservoir-forming process from source rocks to traps and quantitatively predict the resource scale and spatial distribution.

First, the 3D geological framework is established for the sag, and the sag-wide merged 3D seismic data platform is used to precisely interpret the structures and accomplish the top structure maps of key target formations. The spatial distributions of depositional systems, reservoir sands, caprocks, and traps are clarified, and the framework of basic geologic elements is set up. Second, the hydrocarbon generation model (system) is established for the sag, and the precise source rock logging evaluation results are based upon to determine the hydrocarbon generation kinetic parameters of source rocks with different abundance levels. With reference to the hydrocarbon generation kinetic parameters of lacustrine source rocks, and in combination with the organic geochemical characteristics of source rocks, the kinetic parameter model of various source rocks is established and the rational hydrocarbon generation model is proposed. Then, the migration model is established, and the regional sand distribution and reservoir physical property parameters are analyzed according to the results of depositional system study and reservoir appraisal for different layers to finalize the conducting system framework. All types of reservoirs are dissected, and the oil column height together with formation thickness is used to analyze the sealing of caprocks and identify the main factors controlling the reservoir distribution. The hydrocarbon generation chemical kinetics and the hydrocarbon migration and accumulation fluid kinetics are combined to build the migration model, with the hydrocarbon migration pathway and accumulation position restricted with direct evidence of hydrocarbon migration and accumulation; the limited geochemical data are extrapolated to a larger scale to analyze the oil generation and migration system, and calculate the hydrocarbon expulsion quantity and accumulation quantity. Finally, the optimum play fairways are determined.

After the above activities, reservoir-forming modeling is carried out. The results are repeatedly compared to the discovered reservoirs (position, scale, and phase), and the controlling parameters (oil source, trap, reservoir bed, pathway, and caprock) are corrected to finally determine the optimum assemblage for modeling. At last, the sag-wide overall dynamic reservoir-forming modeling is conducted to systematically simulate the hydrocarbon generation, migration, and accumulation, and understand the hydrocarbon accumulation distribution in different ages, horizons, and blocks.

3.4 Creating the New Model of Multiprospect Hydrocarbon Accumulation

Guided by the composite hydrocarbon accumulation theory, the previous exploration in the Bohai Bay Basin was carried out targeting the structural plays and medium-shallow buried-hill top reservoirs, contributing a stage of reserve addition peak. Along with the deepening of exploration, it is increasingly difficult to acquire more discoveries in these plays. In this circumstance, re-exploration for oil-rich sags is proposed. In accordance with the new theories of sub-sag oil accumulation for continental rifts and subtle buried-hill hydrocarbon accumulation, with the support of the sag-wide merged 3D seismic data platform, precise research is conducted on hydrocarbon accumulation conditions, and the multiprospect multitype hydrocarbon accumulation models are established dominantly for stratigraphic-lithologic plays, in order to effectively direct the play selection and exploration deployment.

In the Raoyang Sag, for example, the hydrocarbon accumulation conditions in different plays/structural trends, such as the sub-sag belt, slope belt, and buried-hill belt, are comprehensively analyzed and then the multiprospect hydrocarbon accumulation models are set up. For the Nanmazhuang West-Liuxi sub-sag, models of the delta front sand oil reservoir and the fault grid transportation-fault sand coupling fluvial sand oil reservoir are built. For the Lixian slope, the new model of the platform slope sand up-dipping pinch out and lens sand oil reservoir is proposed. For the subtle buried-hill plays, three buried-hill hydrocarbon accumulation models are put forward, namely, old reservoir and old sealing (Chang 3), red caprock and lateral migration (Ninggu 8 ×), and great mountain-peak accumulation (Hu 8). These models play an important role in guiding the overall research, trap confirmation and favorable lead evaluation for the previously mentioned plays/prospects.

3.5 Multiprospect Overall Preexploration

The multiprospect overall preexploration for re-exploration is neither the local exploration for the purpose of finding the crests of second-order or third-order structure zones nor the precise exploration, which has survived for years, nor the rolling exploration, to extend the reservoirs. Actually, the overall preexploration is designed to identify optimal targets in various plays through overall research, overall cognition, and establishment of new hydrocarbon accumulation models so as to discover and control large-scale