N. K. Thakur

World’s Oil and Natural Gas Scenario

Recent studies suggest that the earths crust may hypothetically hold 6,000 Billion Barrels of oil as its reserves, which also include 3,000 Billion Barrel un-recovered oil resource. However, owing to the complexities in geology associated with the reservoirs one can only indirectly get some inference about the quantum of reserves based on some probabilistic distribution. With 95% probability the world may touch ultimate recovery of 2,248 Billion Barrels. Owing to the non-encouraging scenario of oil production and likely higher cost of oil in future, may lead to search for alternative replacement to oil. The world gas reserves are estimated to the order of 10,000 Trillion Cubic Feet (TCF) out of which only 6,186 TCF are the proven reserves. The projected world natural gas consumption may reach to 158 TCF by 2030. This chapter provides an overview of present oil production, its consumption and future prospects.

Some Facts, Data Analysis, and Examples

The facts about unconventional hydrocarbons are introduced with a discussion on different geographical regions including continental United States and Europe. The complete details of hydraulic fracturing operations are explained including cementing of the “casing shoe”, “kick-off-point”, “well-head”, “perf”, and “proppant”. An example of integrating real geology, geophysics, geomechanics, and petrophysics data for optimization of a horizontal well and multistage fracture stimulation for a tight oil reservoir is discussed. The fundamentals of coal seam gas (CSG) field development, fracture stimulation, and production are explained, and its impact on human health and the environment are touched on. The application of a cross-well seismic modeling approach for very high-resolution imaging, amplitude versus offset (AVO) modeling, to identify thin coal layers is introduced. A full discussion on gas hydrates mapping, including concentration estimation methods and real data examples from different geographical regions, is included. Oil shale reservoir challenges with real data examples and the producing oil−gas ratio, including gas condensate and oil reservoirs, are described. Finally, gas hydrates-related seismic data analysis, full-waveform modeling, mode conversions, seismic indicators including bottom-simulating reflector (BSR), enhanced seismic reflection, gas chimneys, amplitude blanking, hydrate mound, and seismic uplift are explained with real data examples.

Generation of Methane in Earth

This chapter introduces physical conditions for the formation of gas hydrate and their shapes, worldwide distribution, together with the pattern of solubility methane gas in water for given pressure-temperature regime. It then explains the governing processes through chemical, physical reactions under different biological, geological conditions that act on the preserved organic matter in the sediments for the formation of methane in the subsurface earth. By the end of the chapter depth of burial, nature of organic matter, physical, chemical and biological hierarchy that produce methane and have genetically different characteristic are explained. The description of methane derived from decomposition of organic matter and methane that is formed by chemical processes is included. A full discussion on the classification of kerogen that includes Type-I, Type-II, Type-III & Type-IV and formation of methane, coal and oil is included to understand the thermal processes, thermal degradation-cracking of sapropelic organic matter, and humic source.

The Road Ahead and Other Thoughts

The global energy consumption by source is introduced. The difference between conventional and unconventional hydrocarbon reservoirs is described, followed by an explanation of the five main steps of an oil and gas field development project, including discovery, data evaluation, full field development, production, and abandonment. The worldwide occurrence of unconventional hydrocarbons is outlined and the prospects for tight gas reservoirs, shale gas, oil shale/liquid-rich shale, tight oil/shale oil, coal bed methane (CBM)/coal seam gas (CSG), and gas hydrates are discussed in detail. The pillars for subsurface data integration success including rock geometries, rock properties, reservoir modeling, reservoir simulation, strategy, and management are explained. Future trends in advanced quantitative interpretation methods, which are a challenge for unconventional hydrocarbon reservoirs, are highlighted. The key factors that determine the success of unconventional reservoir exploration and development are highlighted.

Tectonics and Gas Hydrates

A global picture of the nature of gas hydrate formations over convergent and divergent continental margins is presented. The morphological, geological, structural, sedimentation, and organic features of gas hydrate formations are broadly explained. The pressure−temperature conditions, geothermal gradient, thermal conductivity conditions for hydrate stability, methane solubility, fluid expulsion, and migration rate of sediments over the continental margins are discussed in detail. The role of porosity, permeability, and polygonal faulting in channelizing fluid flow for hydrates formation is discussed. Finally, gas hydrate-related sediment thickness of existing sedimentary sections, accretionary prisms, axial trench sedimentation, development of decollement, underplating, and young subducting plate convergence rates are reviewed.

Identification to Quantification of Gas Hydrates

In order to meet the world’s energy requirements, identification to quantification of potential energy resources such as gas hydrates is of prime importance. Here we present details of different interpretation techniques and seismic characteristics for drawing inference about the presence of gas hydrates in a region of investigations. The regional identification of gas hydrates could be achieved by travel time inversion schemes. Full waveform modeling is useful in studying the different characteristics, including finer velocity and density estimates of gas hydrates and associated free gas. Seismic attributes are necessary in the direction of predicting gas hydrate saturation. The calculation of seismic attributes can be achieved by the integrated workflows of seismic, geology, pertophysics and rock physics techniques. Modeling for estimating BSR strength in terms of thickness of hydrate/free gas layers with information contained in the velocity estimation is a useful tool for estimating the reflection coefficients, Poisson’s ratios and quantification of gas hydrate resources. This chapter summarizes the prevalent techniques for regional to local mapping of gas hydrates. To this end few synthetic and real data examples of gas hydrate characterizations from different regions of the world are discussed.

Geophysical Surveys and Data Analysis

Gas hydrates occur worldwide in marine sediments and can play a major role in contributing world’s energy requirements. The identification and assessment of gas hydrate volume can be done by different geophysical techniques including various types of seismic surveys, like (2D/3D conventional, ocean bottom seismic, vertical seismic profiling, cross-well seismic and multi-component), well logging, and control source electromagnetic surveys. This chapter provides a brief of various survey designs and optimal survey parameters for gas hydrate exploration. Reflection seismic profiles are useful to construct the compressional velocity (VP) model for hydrate bearing formations and to explore its possible lateral variation and thereby provide possible relevant interpretations in terms of the geology/tectonics of the subsurface earth. Ocean bottom seismic surveys are the key to explore deeper structures and to build the shear velocity model for hydrates. The results of various gas hydrate models together with the field data reveals that seismic methods are able to detect the lower stratigraphic bound of the hydrates as there is no seismic reflection from upper bound and there is no seismic signature within the hydrate stability zone. Another technique for hydrate detection includes well logging (electrical resistivity, gamma ray etc.) which, provide point measurements and provide no information into the lateral distribution of hydrates. Electromagnetic methods for hydrate detection are also feasible but require more field attempts and laboratory studies.

2D traveltime inversion for gas hydrates in Kerala‐Konkan basin, western offshore India

A huge reservoir of carbon resides as methane in clathrate deposits in sediments under the world’s oceans as well as in on-shore sediments in the Arctic. As the search for oil and gas extends to deeper waters, gas hydrates are becoming more of a concern in terms of both safety and as a potential energy source. In this paper, we present the results of 2D travel time inversion on multichannel seismic data of Kerala-Konkan (KK) basin, western offshore India for gas hydrates investigations. The lateral and vertical extent of gas hydrate and free gas zone within the sedimentary cover of the passive KK basin has been mapped.