Unconventional Gas Accumulations Research Articles

Theory and practice of unconventional gas exploration in carrier beds: Insight from the breakthrough of new type of shale gas and tight gas in Sichuan Basin, SW China

Unconventional gas in the Sichuan Basin mainly includes shale gas and tight gas. The development of shale gas is mainly concentrated in the Ordovician Wufeng Formation–Silurian Longmaxi Formation, but has not made any significant breakthrough in the Cambrian Qiongzhusi Formation marine shale regardless of exploration efforts for years. The commercial development of tight sandstone gas is mainly concentrated in the Jurassic Shaximiao Formation, but has not been realized in the widespread and thick Triassic Xujiahe Formation. Depending on the geological characteristics of the Qiongzhusi Formation and Xujiahe Formation, the feedback of old wells was analyzed. Then, combining with the accumulation mechanisms of conventional gas and shale gas, as well as the oil/gas shows during drilling, changes in production and pressure during development, and other characteristics, it was proposed to change the exploration and development strategy from source and reservoir exploration to carrier beds exploration. With the combination of effective source rock, effective carrier beds and effective sandstone or shale as the exploration target, a model of unconventional gas accumulation and enrichment in carrier beds was built. Under the guidance of this study, two significant results have been achieved in practice. First, great breakthrough was made in exploration of the silty shale with low organic matter abundance in the Qiongzhusi Formation, which breaks the traditional approach to prospect shale gas only in organic-rich black shales and realizes a breakthrough in new areas, new layers and new types of shale gas and a transformation of exploration and development of shale gas from single-layer system, Longmaxi Formation, to multi-layer system in the Sichuan Basin. Second, exploration breakthrough and high-efficient development were realized for difficult-to-produce tight sandstone gas reserves in the Xujiahe Formation, which helps address the challenges of low production and unstable production of fracture zones in the Xujiahe Formation, promote the transformation of tight sandstone gas from reserves without production to effective production, and enhance the exploration and development potential of tight sandstone gas.

Unconventional natural gas accumulation system

Unlike conventional natural gas accumulations, unconventional ones have obvious particularity in reservoir forming conditions, enrichment mechanism, distribution modes, prediction method, and exploration technology. Analyzing the reservoir forming process and main controlling factors of unconventional natural gas accumulations under the guide of the hydrocarbon accumulation system theory is of great significance for unconventional natural gas reservoir evaluation and prediction. Based on the reservoir forming mechanism and distribution mode, the unconventional natural gas accumulation systems are divided into six types, including shale gas, coalbed methane, tight carbonate gas, tight sandstone gas, water-soluble gas, and gas hydrate. According to the concept of “element–function–structure” in the hydrocarbon accumulation system theory and the relationship of “gas source rock–migration system–accumulations distribution”, the unconventional gas accumulation system is divided into six “source–location” types. The characteristics of the other five types of unconventional gas accumulation systems except gas hydrate are analyzed. The following results were obtained. (1) Shale gas, coalbed methane and tight carbonate gas belong to an intra-source type unconventional gas accumulation system, characterized by intense gas supply capacity, lack of either a transport system or secondary migration, integration of source, reservoir and caprock, in-situ accumulation, and so on. (2) Tight sandstone gas and water-soluble gas belong to a marginal unconventional gas accumulation system of source rock, characterized by diverse gas origin, near-source accumulation in adjacent layers of source rocks, internal migration within reservoirs, and so on. (3) Basin tectonic evolution, sedimentary environment and late tectonic events control the types of unconventional gas accumulation systems: The Paleozoic marine sediments in South China are conducive to the formation of shale gas and tight carbonate gas accumulation systems; The marine–continental transitional facies deposits of Upper Paleozoic distributed in the South and North China are conducive to the development of coalbed methane, shale gas and tight sandstone gas accumulation systems; In the Middle and East China, Mesozoic–Cenozoic deep-water continental deposits are more conducive to the deep tight sandstone gas and shale gas accumulation systems, while shallow water continental basins are more conducive to the formation of coalbed methane, tight sandstone gas and shale gas accumulation systems. Water-soluble gas is widely found in the Cenozoic shallow-buried areas rich in organic matter or high-pressure reservoirs adjacent to gas source rocks.

The application of volume texture extraction to three-dimensional seismic data – lithofacies structures exploration within the Miocene deposits of the Carpathian Foredeep

There are numerous conventional fields of natural gas in the Carpathian Foredeep, and there is also evidence to suggest that unconventional gas accumulations may occur in this region. The different seismic sig-natures of these geological forms, the small scale of amplitude variation, and the large amount of data make the process of geological interpretation extremely time-consuming. Moreover, the dispersed nature of information in a large block of seismic data increasingly requires automatic, self-learning cognitive processes. Recent developments with Machine Learning have added new capabilities to seismic interpretation, especially to multi-attribute seismic analysis. Each case requires a proper selection of attributes. In this paper, the Grey Level Co-occurrence Matrix method is presented and its two texture attributes Energy and Entropy. Haralick’s two texture parameters were applied to an advanced interpretation of the interval of Miocene deposits in order to discover the subtle geological features hidden between the seismic traces. As a result, a submarine-slope channel system was delineated leading to the discovery of unknown earlier relationships between gas boreholes and the geological environment. The Miocene deposits filling the Carpathian Foredeep, due to their lithological and facies diversity, provide excellent conditions for testing and implementing Machine Learning techniques. The presented texture attributes are the desired input components for self-learning systems for seismic facies classification.

Możliwości występowania gazu zamkniętego w piaskowcach środkowego kambru zachodniej części obniżenia bałtyckiego (wyniesienie Łeby) – wstępna analiza na podstawie danych petrofizycznych i geofizyki otworowej

Prospects of tight gas occurrence in the Middle Cambrian sandstones in the western part of the Baltic Depression (Łeba Elevation) – introductory analysis based on petrophysical and well logging data. A b s t r a c t. The main purpose of this article is an approximation of petrophysical parameters in selected borehole sections in the Łeba Elevation. The parameters are among the most important factors affecting the possibility of unconventional gas accumulation. The other factors explaining the possibility of tight gas accumulations in the Middle Cambrian sandstones of the Łeba Elevation are hydrocarbon shows in cores and drilling mud and the presence of conventional hydrocarbon fields. The potential hydrocarbon resources are related to the Debki and Bialogora Formations. They are composed of fine-grained sandstones. The thickness of the Debki Formation ranges up to 100 m. The tickness of the Bialogora Formation does not exceed 10 m. The analysis of petrophysical parameters of these formations indicates that their tight gas saturation has a layered character. The Debki Formation shows a better potential for tight gas accumulation.

Miocene Biogas Generation System in the Carpathian Foredeep (SE Poland): A Basin Modeling Study to Assess the Potential of Unconventional Mudstone Reservoirs

This paper presents the results of a research project aimed at evaluating the unconventional natural gas potential of the autochthonous Miocene sediments in the Polish part of the Carpathian Foredeep. The primary objective of the study was to re-evaluate the biogenic gas generation system within Miocene sediments, paying special attention to unconventional gas resources accumulated in tight mudstone formations. The four-dimensional (4D) petroleum system modeling method (PetroMod software) was used to reconstruct the basin geometry and three-dimensional (3D) evolution through a geological timescale, in particular the progress of gas generation, migration, and accumulation processes, as well as their consequences for gas exploration and development. Special attention was paid to the dynamics of gas generation processes and the advancement of sediment compaction and their time dependence, as well as to the progress and outcomes of gas migration and accumulation processes. The results indicate significant potential for unconventional gas accumulations in mudstone reservoirs. However, part of the biogenic gas resources occurs in a dispersed form. Analysis of the dynamics of biogenic gas generation and accumulation conducted on a basin scale and within particular sedimentary complexes and depth intervals allowed an indication of the premises regarding the most favorable zones for mudstone–claystone reservoir exploration.

Theory, technology and prospects of conventional and unconventional natural gas

Abstract The development of natural gas in China has entered a golden and leap-forward stage, which is a necessary bridge to clean energy. This in-depth study on the status quo, theory, technology and prospect of natural gas development shows: (1) The global remaining proven recoverable reserves of natural gas are 186×1012 m3 and the reserves-production ratio is 52.4, indicating a solid resource base for long-term and rapid development. (2) Ten formation and distribution laws of conventional and unconventional natural gas reservoirs have been proposed. In terms of exploration geology, the theory of conventional “monolithic” giant gas fields with different gas sources, and an unconventional gas accumulation theory with continuous distribution of “sweet areas” in different lithologic reservoirs have been established; in terms of development geology, a development theory of conventional structural gas reservoirs is oriented to “controlling water intrusion”, while a development theory of unconventional gas is concentrated on man-made gas reservoirs. (3) With the geological resources (excluding hydrates) of 210×1012 m3 and the total proven rate of the resources less than 2% at present, the natural gas in China will see a constant increase in reserve and production; by 2030, the proven geological reserves of natural gas are expected to reach about (6 000− 7000)×108 m3 the production of conventional and unconventional natural gas each will reach about 1 000×108 m3 and the gas consumption will reach 5500×108 m3. The dependence on imported natural gas may be 64% by 2030, and 70% by 2050. (4) Ten measures for future development of natural gas have been proposed, including strengthening exploration in large-scale resource areas, increasing the development benefits of unconventional gas, and enhancing the peak adjusting capacity of gas storage and scale construction of liquified natural gas.

Continuous unconventional natural gas accumulations of Carboniferous-Permian coal-bearing strata in the Linxing area, northeastern Ordos basin, China

Based on results of laboratory tests and a study of exploration wells, we investigate Carboniferous–Permian unconventional natural gases in lateral extension and vertical sequences in northeastern Ordos basin, China. The primary objectives and techniques are (1) hydrocarbon generation analysis based on Rock-Eval pyrolysis and microscopic observations; (2) reservoir characterization based on depth and thickness extension, nuclear magnetic resonance (NMR) and He porosity/permeability; and (3) gas accumulation analysis based on well cross-sections, field well tests and thermal evolution history.Shale and coal are rich in organic matter and favorable for gas generation, with thermal maturity of approximately 1.4% and shales of type II2 to III kerogen. The gas contents of coal vary from 5 to 20 m3/t with total thicknesses in the Taiyuan and Shanxi Formations of 8–18 m and 4–14 m, respectively, and burial depth increasing from 200 to 2000 m from east to west. Shale and sandstone are also greatly distributed with average total thickness greater than 25 m in both Shanxi and Taiyuan Formations. Porosity is poor for all potential reservoirs, with those of coal, shale, and sandstone being 1.0%–4.9%, 1.0%–1.5%, and lower than 10%, respectively. Three types of unconventional gas accumulations can be distinguished based on differences in source-reservoir combination. The inner source accumulation is composed of coalbed methane (CBM) and tight gas (TG) with direct contact between coal and sandstone, and the strata pressure coefficient (σ) generally varies from 0.8 to 1.0. Close source accumulation is a type of typical continuous TG reservoir with tight sandstone closely related to the source rocks and σ ranging from 0.7 to 1.0. Near source accumulation is relatively distant from source rocks and relies on seal conditions, with σ ranging from 0.4 to 0.7. Due to the gently dipping strata, the eastern area is relatively favorable for CBM development, and the western area is favorable for TG development. Furthermore, shale gas should also be considered in the western area due to good hydrogen generation potential and preservation conditions. Our results are beneficial for the co-production of unconventional natural gas in the entire Ordos basin and other coal-bearing basins.

Contribution and significance of dispersed liquid hydrocarbons to reservoir formation

Abstract Oil expulsion rate of different types of source rock in geologic settings are estimated by modeling hydrocarbon generation and expulsion and examining the geological sections of different source-reservoir combinations. The major gas generation peak of oil-cracked gas kitchen is determined by study on the gas generation timing of dispersed liquid hydrocarbon inside and outside source rock and ancient oil reservoirs, and the factors affecting the gas generation timing. A “five-step method” based on the origin method is set up to quantitatively evaluate the cracked gas generated by dispersed liquid hydrocarbons. The hydrocarbon generation and expulsion experiment of different types of source rock, the study on oil expulsion rate of different source-reservoir combinations, analysis of rock pyrolysis parameters of reservoir rock all show that there is a large amount of liquid hydrocarbons retained in the source rock, and the feature on the whole is the oil expulsion rate decreases with the reducing of total organic carbon (TOC). The expulsion rate of source rock with TOC of less than 2% is below 50% in general during the liquid-window stage, while that of alternating sandstone and mudstone and of thick mudstone, with TOC of 2%−4%, are 60% and 30% respectively. Affected by the ancient landform, the expelled liquid hydrocarbons accumulated in different abundance in formations, with dispersed or semi-dispersed and semi-gathered form, and they are called dispersed liquid hydrocarbons outside source rock. When buried deeper, both dispersed liquid hydrocarbons inside and outside the source can be cracked into natural gas to form conventional and unconventional gas accumulations. Exploration practices in the Sichuan and Tarim basins have proven that the cracked gas generated by dispersed, semi-dispersed and semi-accumulated, and accumulated liquid hydrocarbons is an important source of deep natural gas in China, and takes an important position in gas exploration.

Similarities and differences between natural gas accumulations in Sulige gas field in Ordos basin and Xujiahe gas field in Central Sichuan Basin

Both Sulige gas field in the Ordos Basin and Xujiahe gas field in Central Sichuan Basin are large low-porosity and low-permeability gas fields composed of conventional gas reservoirs and unconventional gas accumulations, representing typical cases of low-grade natural gas accumulations in China. Studies show there are both similarities and differences between these two fields in terms of forming conditions, mechanism and distribution. The similarities include: (1) large-scale development of accumulation conditions and close source-reservoir contacts, providing basis for large-scale accumulations; (2) large-scale energy storage during source kitchen burial and large-scale gas expulsion during uplift, providing important conditions for large-scale natural gas accumulation; (3) coexisting volume flow and diffusion flow, giving rise to the co-existence of conventional gas reservoirs and unconventional gas accumulations; and (4) clustered lithologic gas accumulations on a large scale. The differences lie in: (1) different source-reservoir-seal assemblages, giving birth to two kinds of accumulations, i.e. large area accumulations and large-scale patchy accumulations; (2) different source kitchen continuity and gas supply potentials, leading to different enriched gas-bearing layer series and different abundances. Timely understanding of the similarities and differences between large-scale natural gas accumulations with low porosity and permeability in these two basins would be conducive to avoid risks and improve exploration success ratio and benefits.

Determining the source and genetic fingerprint of natural gases using noble gas geochemistry: A northern Appalachian Basin case study

Silurian and Devonian natural gas reservoirs present within New York state represent an example of unconventional gas accumulations within the northern Appalachian Basin. These unconventional energy resources, previously thought to be noneconomically viable, have come into play following advances in drilling (i.e., horizontal drilling) and extraction (i.e., hydraulic fracturing) capabilities. Therefore, efforts to understand these and other domestic and global natural gas reserves have recently increased. The suspicion of fugitive mass migration issues within current Appalachian production fields has catalyzed the need to develop a greater understanding of the genetic grouping (source) and migrational history of natural gases in this area. We introduce new noble gas data in the context of published hydrocarbon carbon (C1,C2+) (13C) data to explore the genesis of thermogenic gases in the Appalachian Basin. This study includes natural gases from two distinct genetic groups: group 1, Upper Devonian (Marcellus shale and Canadaway Group) gases generated in situ, characterized by early mature (13C[C1 C2][13C113C2]: –9), isotopically light methane, with low (4He) (average, 1 103 cc/cc) elevated 4He/40Ar and 21Ne/40Ar (where the asterisk denotes excess radiogenic or nucleogenic production beyond the atmospheric ratio), and a variable, atmospherically (air-saturated–water) derived noble gas component; and group 2, a migratory natural gas that emanated from Lower Ordovician source rocks (i.e., most likely, Middle Ordovician Trenton or Black River group) that is currently hosted primarily in Lower Silurian sands (i.e., Medina or Clinton group) characterized by isotopically heavy, mature methane (13C[C1 – C2] [13C113C2]: 3), with high (4He) (average, 1.85 103 cc/cc) 4He/40Ar and 21Ne/40Ar near crustal production levels and elevated crustal noble gas content (enriched 4He, 21Ne, 40Ar). Because the release of each crustal noble gas (i.e., He, Ne, Ar) from mineral grains in the shale matrix is regulated by temperature, natural gases obtain and retain a record of the thermal conditions of the source rock. Therefore, noble gases constitute a valuable technique for distinguishing the genetic source and post-genetic processes of natural gases.

Study on the Effect of Source-Contacting Gas Accumulations upon Abnormal Pressures in Western Sichuan Depression

The migration and accumulation of typical source-contacting gas, also called basin-centered gas, follow the piston principle that it generates superpressures essentially. In the tight sand reservoir, the formation water cannot exchange sufficiently, which maintains higher pressure in gas reservoirs compared with conventional reservoirs during tectonic uplift or subsidences. The western Sichuan depression is one of the earliest basins in China being researched for source-contacting gas, where the regional overpressure is the direct product and the sign of the accumulation of source-contacting gas. Besides the effect from the accumulation of source-contacting gas, the regional overpressure in the western Sichuan depression is also directly related with undercompaction and regional tectonic uplift before and after the accumulation of source-contacting gas, respectively. Present regional overpressure in the western Sichuan depression is attributed to three phases of tectonic movements and generations of abnormal pressure thrice due to the preferable obturation of the unconventional gas accumulation systems. The multiple pressures were believed to cumulate from undercompaction and hydrocarbon generation before the Late Jurassic period, from gas generation and accumulation of source-contacting gas in the Late Jurassic to Eogene period, and cumulation of multiple pressures after the Late Eogene is attributed to regional uplift. Using section configuration and calculation, the coefficients of abnormal pressures in the western Sichuan depression can be divided as follow: the coefficients of abnormal pressure due to undercompaction and regional uplift is 1.42 and the rest over-pressures is attributed to the accumulation of source-contacting gas. According to the isolines 1.42 of pressure coefficients, the distribution of source-contacting gas accumulations can be approximately predicted.

Unconventional Gas Traps: Low Permeability Sands and Gas Accumulations

This paper documents the presence of commercial gas production from tight, unconventional, pervasive gas accumulations in North America. The work consisted of two main phases: (i) library research and data collection; and (ii) application of both a reservoir engineering model and a basin analysis model to determine the origins of tight gas sands. The Wattenburg Field in the syncline of the Denver Basin, the Almond Sand trend in the Eastern Green River Basin, and the Canyon Sands of the Ozona-Sonora trend of the Val Verde Basin, all represent recent development projects in the USA in tight gas sands, while the Deep Basin trend of the Western Alberta Basin, including the giant Elmworth Field which was discovered in 1976, yielded a rapid development of this trend in the early 1980's. The main products of the first phase of this work are the Field Summary Sheets of Appendix A, and the Spreadsheets of Appendix B, which collect and categorize the data about each of the fields and reservoirs. The Field Summary Sheets gather the data on fields and allow the comparison of their main characteristics, with concentration on the geological characteristics of the 40 typical reservoirs examined in the study. This database was used to try to make sense of the often conflicting values of the parameters that these fields have in common. The main text of the paper attempts to summarize and synthesize this database. In the application to the Elmworth Field, the largest gas field of the Western Canada sedimentary Deep Basin, gas found in stratigraphic traps lies downdip from water, with no known permeability barrier between. A two dimensional quantitative analysis of the basin was undertaken in order to simulate such unconventional entrapment. The modeling shows how it is possible to accumulate gas in the low permeability sand reservoirs of the Lower Cretaceous. It appears that the Elmworth basin is a system in dynamical evolution, where gas loss due to migration is partly compensated for by contemporary gas generation. While the on-going gas generation is highly influenced by the recent paleoheat flow, the early migration has been controlled by cementation which, by lowering the permeability, reduces the gas loss with time, so that significant gas trapping can be obtained today. Overall, the accumulations are described in terms of an impedance trap where the presence of gas is a transient phenomena in terms of geologic time. The free flow of gas from the source rock to surficial escape is impeded by the tight sands, to the point where a commercial volume of gas is trapped in the basinal syncline or on monoclinal dip on the basin flanks. This paper describes how and why this trapping mechanism works, and what the accumulations have in common. The work presents a set of conditions leading to such unconventional gas accumulations and discusses the parameters influencing the total amount of gas accumulated at present day; characteristics of the sediments, such as porosity and permeability behavior through time, cementation parameters, amount of sediments removed during erosion events, or heat flow variation with time, are also bracketed in order that gas accumulations predicted are in agreement with those observed today. The underpressure due to gas saturated formations is also considered.

Modelling Unconventional Gas Traps: Elmworth Field, Alberta, Canada

The Elmworth Field is the largest gas field of the Western Canada sedimentary Deep Basin. Gas found in stratigraphic traps lies downdip from water, with no known permeability barrier between. A two dimensional quantitative analysis of the basin was undertaken in order to simulate such unconventional entrapment. The modelling shows how it is possible to accumulate gas in the low permeability sand reservoirs of the Lower Cretaceous. It appears that the Elmworth basin is a system in dynamical evolution, where gas loss due to migration is partly compensated for by contemporary gas generation. While the on-going gas generation is highly influenced by the recent paleoheat flow, the early migration has been controlled by cementation which, by lowering the permeability, reduces the gas loss with time, so that significant gas trapping can be obtained today. This work presents a set of conditions leading to such unconventional gas accumulations and discusses the parameters influencing the total amount of gas accumulated at present day; characteristics of the sediments, such as porosity and permeability behavior through time, cementation parameters, amount of sediments removed during the Paleocene erosion, or heat flow variation with time, are also bracketed in order that gas accumulations predicted are in agreement with those observed today. The inherent underpressure due to gas saturated formations is also considered.