Hydrocarbon Fluid Inclusions in Petroliferous Basins

By Vivekanandan Nandakumar and J.L. Jayanthi

Hydrocarbon Fluid Inclusions in Petroliferous Basins trains readers to detect Hydrocarbon Fluid Inclusions (HCFIs) in sedimentary rocks, particularly the wafer preparation techniques to visualize HCFIs, its distinction from aqueous inclusions, petrographic approaches to HCFIs, microthermometric observations on HCFIs, fluorescence emission spectra and Raman spectra of HCFIs, and their interpretations for the petroleum industry. The book features case studies from the Mumbai and Kerala Konkan Basins of the Western Offshore of India - two representative basins where new, non-destructive, fluid inclusion techniques were tested. This book is essential reading for students of petroleum geology and those working in exploration in the oil and gas industry.

• Helps readers to identify Hydrocarbon Fluid Inclusions (HCFIs) in sedimentary basins
• Covers how to determine the oil window, API gravity and chemical constituents in HCFIs
• Includes case studies on key offshore basins

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 24, 2021
• ISBN: 9780128174173

Vivekanandan Nandakumar

Dr. V. Nandakumar is a geoscientist in NCESS, India. He has vast expertise in fluid inclusion work, both in sedimentary and high-grade metamorphic rocks. Dr. V. Nandakumar has more than 28 years of experience in fluid inclusions studies, beginning from his MSc thesis on fluid inclusions in igneous and hydrothermal systems. As a continuation to his MSc thesis, his PhD work was also on fluid inclusions in cordierite bearing granulites. He also collaborated with the Oil and Natural Gas Corporation (ONGC), Government of India to study fluid inclusions from the petroliferous basins and is developing techniques for oil exploration.

Book preview

Chapter 1: Introduction to fluid inclusions

V. Nandakumara; J.L. Jayanthib    a Scientist - G, National Facility for Geofluids Research and Raman Analysis, National Centre for Earth Science Studies (NCESS), Thiruvananthapuram, Kerala, India

b Project Scientist - C, National Facility for Geofluids Research and Raman Analysis, National Centre for Earth Science Studies (NCESS), Thiruvananthapuram, Kerala, India

Abstract

Fluid inclusions are direct samples of palaeofluids that interacted with different stratas of the Earth's crust and are the store house of information on the pressure–temperature conditions, density, and composition that lead to an understanding of the evolutionary history of rocks. Hydrocarbon-bearing fluid inclusions (HCFIs) are crucially important since they are hidden petroleum shows that store potential information on the digenetic-catagenetic environments that are significant to the petroleum exploration industry.

Keywords

Fluid inclusions; Hydrocarbon-bearing fluid inclusions; Genetic and compositional classification of fluid inclusions; Significance of geofluids; Importance of HCFIs

1.1: Entrapped fluids: Fluid inclusions

When a crystal grows in the presence of a fluid phase, some of the fluid may get trapped in the imperfections in the growing crystals to form fluid inclusions. The trapped fluid may be liquid, vapor, or supercritical fluid and the fluid composition may include pure water, brines of various salinity, gas or gas-bearing liquids, petroleum, silicate, sulfides, carbon melts, etc. Thus, fluid inclusions represent the trapped portions of gas, liquid, or melts from which the crystal had grown and could be used to establish the environment in which a rock or mineral might be formed. In the literature, the term fluid inclusion has been used for inclusions that are trapped as fluid and remains as fluid at surface temperature (Roedder, 1984). In the early 19th century, eminent British scientists Sir Humphrey Davy and Sir David Brewster published fascinating accounts of very large fluid inclusions in quartz. But it was the pioneering work of Henry Clifton Sorby in the mid-1800s following the rapid development of optical microscopy that elevated the status of fluid inclusions from simple objects of curiosity to that of considerable scientific merit and importance.

After crystallization, the minerals of almost all terrestrial sedimentary, metamorphic, and igneous rocks get fractured one or more times and these fractures could have been healed in the presence of liquid or gaseous fluids. During these processes of crystal growth and fracture healing, small quantities of the surrounding fluid medium get trapped as fluid inclusion in the host crystal. The subject of fluid inclusions in sedimentary systems must include all of those systems forming at the Earth's surface and those that extend more deeply below the surface until conditions leave the digenetic-catagenetic realm probably at temperatures below about 140 °C. This would include evaporates formed from evaporation of saline surface and groundwater, and products of physical and chemical reactions in the subaerial realm where a multitude of fluid exists including oil, gas, and aqueous fluids of various origin.

Geologically, fluid inclusions are small voids that contain a variety of liquids, which are often found in natural minerals and rocks. They are regarded as small sealed vials, often less than 10 μm in size that host fossil fluids that exist when the minerals grew or healed after fracture formation (Roedder, 1984; Goldstein and Reynolds, 1994). The composition of fluid inclusions varies considerably, and may comprise liquid, solid, and/or gaseous phases, depending on the fluid source and pressure–temperature conditions experienced. These phases commonly include water, dissolved gases, and salts; in extreme cases, inclusions may host daughter minerals, high-pressure vapor phases, and complex organic mixtures. Among these, the inclusions, which contain hydrocarbon fluids, which originated from petroleum that once migrated through the rocks before becoming trapped, are of special interest to the petroleum industry (Goldstein, 2001; Munz, 2001). Fig. 1.1 shows some aqueous and hydrocarbon-bearing fluid inclusions (HCFIs) from the Mumbai offshore basin, India.

Fig. 1.1

Fig. 1.1 Examples of multiphase (A), biphase aqueous (C, D, F) and hydrocarbon-bearing fluid inclusions (B, E) from Mumbai offshore basin, India.

1.2: Significance of geofluids

Fluids play a vital role in virtually all crustal and mantle processes. The circulation of fluids has important effects on the transport of chemical constituents and heat and are the principal contributors in the formation of hydrothermal ore deposits. The mechanisms by which crustal rocks deform are strongly influenced by the presence of water, as well as by the pore-fluid pressure. It has also been suggested that fluids play a very significant role in earthquakes. On a broader scale, pore-fluid pressure influences the mechanical processes that control rock deformation in accretionary wedges associated with subduction zones. The volume of fluids carried to deeper levels in subduction zones appears to influence the rate and depth of melting and determines the site of volcanism in the overlying plate (The Role of Fluids in Crustal Processes, 1990). Water and other geofluids play an important role in the geochemical and archeological evolution of the Earth and other planetary bodies in the solar system. These fluids are responsible for the formation of hydrothermal mineral deposits, affect eruption behavior in volcanic systems and the geophysical properties of the mantle, and significantly affect the way in which rocks deform and fracture. Water is required for life to develop and survive and the search for life beyond Earth is naturally a search for water in the solar system and beyond (Bodnar, 2005). The minerals of many meteoritic and lunar samples and of terrestrial igneous rocks have grown from fluid silicate melts. All crystals in all terrestrial and extraterrestrial samples have grown from fluids with an exception of those crystals in metamorphic samples that have grown in the solid state. New crystals in many sedimentary and some metamorphic rocks and in almost all ore deposits are formed from aqueous fluid containing various solutes.

Aqueous fluids play a key role in mass transfer processes in the Earth, including the generation of magmas in the Earth's mantle above subduction zones, the release of fluids from crystalline magmas, the production and migration of fluids during mountain building, the formation of hydrothermal ore deposits, and the interaction of fluids released from the deep Earth with the hydrosphere and atmosphere. As the only topmost part of the Earth's crust is accessible to direct sampling of fluids (i.e., discharge from geothermal systems and deep drill holes), most of the information we have about deep Earth fluids comes from studying palaeo-fluids that are preserved as fluid inclusion in minerals.

Important fluids in the sedimentary realm include atmospheric gases, freshwater of meteoric origin, lake water, seawater, mixed water, evaporated water, formation waters deep in basins, oil, and natural gas. Preserving a record of the distribution and composition of these fluids from the past should contribute significantly to the studies of paleoclimate and global-change research, essential for improving understanding of diagenetic-catagenetic systems and can provide useful information in petroleum geology.

1.3: Fluids of the sedimentary realm

Freshwater of meteoric origin is the most important fluid associated with the sedimentary realm and includes freshwater precipitating on sediment and soil surfaces as rain water, freshwater from lakes and ground water, and water of meteoric derivations, compositionally modified through rock water interaction and evaporation. Seawater is another fluid that is common near the surface of the earth. Today, this fluid is relatively uniform in composition and maintains salinity near 35 ppt. But in the past, the total salinity might have varied, isotopic composition varied, and ratios of major, minor, and trace ions may have been different from what they are today. The chemistry of seawater may have changed so significantly through time that even the major carbonate minerals precipitating from it have changed through time giving rise to ancient seas dominated by aragonite/high-Mg calcite precipitation and seas dominated by calcite precipitation. This variation in seawater chemistry is further supported by secular variation in the composition of evaporate minerals, with seawater of one age giving rise to Mg-rich bitterness and seawater of another age giving rise to K-rich bitterness. Understanding secular variations in the composition of seawater is important as a paleoclimate indicator, for understanding the evolution of organisms, and is also important for understanding the paleoecology of ancient environments, tectonic controls on global cycling of chemical constituents in nature, another digenetic effect of seawater composition in carbonate sediments (Goldstein and Reynolds, 1994). Seawater that has been modified through evaporation or meteoric dilution falls into another class of fluids important in sedimentary systems. Brackish waters, created by dilution of seawater with meteoric water, are important digenetic fluids, which may be responsible for dissolution of carbonate minerals and precipitation of calcite and dolomitization (Land, 1973; Goldstein and Reynolds, 1994). Seawater that has been modified through evaporation is a diagenetically reactive fluid that may be important in precipitating calcite, dolomite, and evaporates.

As one moves more deeply into the sedimentary sections, a wide variety of subsurface fluids may be encountered, along with the temperature and pressure increase normally encountered along with the geothermal gradient. Aqueous fluids ultimately may have been derived from meteoric and marine fluids, but typically, these have been altered so extensively through processes of evaporation at the surface, evaporate dissolution, or other type of rock-water interaction that they have achieved compositions significantly different from their parent fluids. These basinal fluids typically are very salty, may achieve concentrations well above those of marine fluids, and contain ion ratios that reflect their evolution. Many of these fluids are quite important for precipitation of digenetic minerals such as feldspar, quartz, anhydrite, calcite, and dolomite. In addition to aqueous systems, the subsurface may contain petroleum and various compositions of natural gas derived from organic matter.

1.4: Fluid inclusions in sedimentary and diagenetic systems

Detrital quartz grains from sediments often contain abundant fluid inclusions but these are usually unrelated to the fluids developed during burial and compaction of the sediment. Fluid inclusions related to diagenetic or low-grade metamorphic fluid processes are best preserved in the larger veins, vugs, geodes, and concretions sometimes present in these rocks. Diagenetic quartz and carbonate overgrowths and cements in medium to coarse grain sediments should, in theory, also contain fluid inclusions. Fluid dynamics in sedimentary basins is of tremendous interest, both from a scientific and an economic point of view. Integration between fluid inclusion and present-day fluid data provides the time aspect necessary for the reconstruction of fluid flow paths, and can be used for mapping fluid dynamics both on a regional basin scale and on the local scale of petroleum reservoirs. Aqueous fluids dominate in sedimentary systems, and actively participate in diagenetic processes (Goldstein, 2001).

Diagenetic reactions are important in controlling porosity and permeability in oil and gas reservoirs. Diagenesis of sedimentary rocks and carbonates can be closely linked to origin of the fluid responsible for digenetic alteration. Diagenesis can even be linked to the surface environment. Most sediments are deposited by marine fluids at the Earth's surface temperatures. Common diagenetic phases known to precipitate from seawater include fine grained cements of Mg-calcite or aragonite compositions and coarser Mg-calcite and aragonite cements with a variety of morphologies. Low-Mg calcite and dolomite are also known from these systems. When seawater is mixed with freshwater of meteoric origin, there is some evidence for dissolution of carbonate minerals as well as evidence for precipitation low-Mg calcite in some settings and replacement of carbonate minerals with dolomite. At and below the water table, meteoric fluids may mix with other meteoric fluids of differing CO2 content, encouraging dissolution of carbonate minerals. Dissolution of unstable carbonate minerals and out gassing of CO2 encourages precipitation of calcite in this setting. In all of the above settings, minor silicate digenesis is also possible and is greatly dependent on the composition of the pore fluids and their modification by chemical components provided through interaction with the sediment.

As one goes deeper into the sedimentary section into a burial setting, or as warm fluids from deeper in a basin are injected towards the surface, many diagenetic reactions are driven by changing temperature. Among these is dissolution of minerals as well as precipitation of feldspar, quartz, and carbonates. Therefore, a sequence of diagenetic phases in ancient rock, determining the diagenetic environment of its precipitation is determined by two or three basic characteristic of the fluid that precipitated it—salinity, temperature, and pressure. These three parameters are those most easily determined from fluid inclusion analysis of the right suit of fluid inclusions, and the analysis of fluid inclusions in diagenetic minerals is probably the most unambiguous method for determining diagenetic history. Most fluids in the diagenetic realm are incredibly complex in composition. One of the best examples of this is seawater, a highly concentrated fluid (about 3.5 wt% NaCl) that has major amounts of Na+, Mg² +, Ca² +, K+, Sr² +, Cl−, SO4² −, HCO3−, Br−, F−, and B and minor amounts of many other elements. In addition, the composition of organic and inorganic gases and hydrocarbon liquids is also quite