Further comprehension of natural gas accumulation, distribution, and prediction prospects in China

In-depth research reveals that the natural gas accumulation and distribution are characterized by cycle, sequence, equilibrium, traceability, and multi-stage. To be specific, every geotectonic cycle represents a gas reservoir forming system where natural gas is generated, migrated, accumulated, and formed into a reservoir in a certain play. Essentially, hydrocarbon accumulation occurs when migration force and resistance reach an equilibrium. In this situation, the closer to the source rock, the higher the accumulation efficiency is. Historically, reservoirs were formed in multiple phases. Moreover, zones in source rocks and adjacent to source rocks, unconformity belts, and faulted anticline belts are favorable areas to finding large gas fields. Apart from the common unconformity belts and faulted anticline belts, in-source and near-source zones should be considered as critical targets for future exploration. Subsequent exploration should focus on Upper Palaeozoic in the southeastern Ordos Basin, Triassic in southwestern Sichuan Basin, Jurassic in the northern section of the Kuqa Depression and other zones where no great breakthroughs have been made. Keywords: Large gas field, Distribution characteristics, Potential zone, Prospec

Significant advancement in geological theories and new discoveries of natural gas in China since the 11th Five-Year Plan

The development of geologic theories and exploration findings of natural gas in China supplement each other. Since the 11th Five-Year Plan in 2006–2010, geologic theories of natural gas in China has achieved notable advancement in many aspects, of which, are mainly reflected in the following seven aspects. Among them, there are two research progresses in the basic geological theory enumerated as follows. (1) The formation mechanisms of three types of natural gas that have been studied broadly including highly evolved coal-based source rocks, crude oil pyrolysis gas, and biogas. The cracked gas mode of coal-based source rocks, whole process hydrocarbon-generating mode of humus-type organic matter, and continuous biogas generation mode have been thoroughly advanced. (2) The theory of genetic identification between crude oil pyrolysis gas and kerogen pyrolysis gas, aggregated crude oil pyrolysis gas and dispersed crude oil pyrolysis gas, organic and inorganic gases, coal-type gas and oil origin gas, has been enriched extensively. There are five theoretical advances in the field of hydrocarbon accumulation in large gas fields: (1) the theory of hydrocarbon accumulation in ancient carbonate rock, “five paleo-structures control accumulation”, has been proposed innovatively; (2) the accumulation theory of tight sandstone gas in craton basins, foreland basins, and rift basins have been well-established; (3) the accumulation mode of “three-micro conveying, near-source enrichment, and sustained preservation” for ultra-deep and weak deformation zones has been established; (4) the accumulation theory of volcanic gas reserves in rift basins with basic elements of hydrocarbon generation troughs has been established and improved; (5) lastly, the accumulation theory of offshore high-temperature, overpressure, and deepwater gas were methodically deepened. The development of geologic theories of natural gas has promoted many new exploration discoveries. The accumulation theory in ancient carbonate reservoirs paved the way for the exploration of Anyue gas field in the Sichuan Basin, the largest single reserve in China. The new understanding of tight sandstone gas accumulation in the foreland thrust belt helped the first gas field discovery in the size 1 × 1012 m3 in an ultra-deep layer in the Kuqa Depression. The accumulation theory of ultra-deep reef reservoirs has guided the exploration of Yuanba gas field, the deepest-buried reef gas field in China. The theory of offshore hydrocarbon accumulation has led to remarkable discoveries in the South China Sea. Some of the said discoveries are the Dongfang 13-2 gas field, the largest gas field in China located in self-supported areas, and the Lingshui 17-2 gas field, which is a hundred billion cubic meters in size and is located in the deepwater exploration field. Keywords: Natural gas, Basic geological theory, Mechanism of gas generation, Accumulation of large gas fields, New exploration finding

Accumulation mechanism of tight sandstone gas in low gas generation intensity area

Before 11th five-year plan, geologists proposed the viewpoint that the gas generation intensity that's more than 20 × 108 m3/km2 was an important condition for forming conventional large gas fields. However, recent exploration findings indicate that large-area tight sandstone gas that has a gas generation intensity of less than 20 × 108 m3/km2 can still form reserves. This is an area worth exploring. Through innovative accumulation simulation, microscopic pore throat analysis of reservoirs, and dissection of typical gas reservoirs, several factors have been established, including the comprehensive evaluation models involving gas charging pressure, reservoir physical properties, and lower gas generation limit. In addition, the paper has made it certain that the tight sandstone gas in a low gas generation intensity area has accumulation characteristics such as “partial water displacing, long-term gas supply, gas control by tight reservoirs of scale, gas abundance control by physical properties, combined control and enrichment of dominant resources, etc.”, and has proposed the viewpoint that this area has the accumulation mechanism that of a “non-dominant transportation, long-term continuous charging, reservoir controlling by physical property difference, and enrichment in partial sweet spots” and shows discontinuous “patchy distribution” on the plane. This is of much significance to fine exploration and development of the trillion cubic meter resources of the low gas generation intensity areas in the west of Sulige gas field

Quantitative research on tight oil microscopic state of Chang 7 Member of Triassic Yanchang Formation in Ordos Basin, NW China

With nuclear magnetic resonance (NMR) and micrometer-nanometer CT Scanning technology, quantitative research was carried out of the micro-existing state of tight oil in the Chang7 Member of the Triassic Yanchang Formation in the Ordos Basin. The total reserves of the tight oil in the reservoirs were determined using NMR technology. The initial oil saturation was measured as 63.99%. The 2-D slice images of the tight oil reservoirs were obtained using CT Scanning technology, and the 3-D images were acquired after digital synthesis processing. Accordingly, the existence states of the tight oil in the reservoirs are divided into six types, namely thin film form, cluster form, throat form, emulsion form, particle form and isolation form. It is found by quantitative calculation of the contents of tight oil in different existence states of tight oil that the emulsion form and thin film form are the main existence states, which account for 70% of the total amount of the tight oil in the reservoirs, followed by the cluster and particle forms. The contents of isolation form and throat form tight oil are low. The contents of tight oil in various existence states are related to initial water saturation, clay mineral content and pores structure of the reservoirs. Key words: tight oil, microscopic state, quantitative research, Ordos Basin, Triassic Yanchang Formatio

Distribution laws of large gas fields and further exploration orientation and targets in China

With the rapid expansion and extension of natural gas exploration and development, it is more and more difficult to discover large-scale reserves in China. To intensify the research on new natural gas exploration domains, we reviewed the progress and trend of natural gas exploration and analyzed the main areas with large-scale proved gas in place (GIP). Then, based on a statistic analysis of large gas fields in China as well as their hydrocarbon accumulation characteristics, their genetic systems were classified and each system's distribution law was also identified. Some research results were obtained. First, carbonate paleo-uplifts of cratonic basins, tight sandstones of extensive gentle slopes and thrust structures of foreland basins are the main areas with large-scale proved GIP in China. Second, there are five genetic systems for large gas fields, i.e. cratonic rift and paleo-uplift (A), stable slope of low-angle open lake (B), thrust structure of piedmont fault depression (C), faulted uplift and igneous rock of intracontinental pull-apart fault depression (D), and anticline structure of epicontinental strike–slip fault depression (E), and one genetic system (F) for unconventional gas, i.e. adsorption and accumulation in nano-scale space. Third, there is one core genetic system for large conventional gas fields in each geologic cycle. Fourth, two-level accumulation, i.e. no migration inside the source and large-scale transportation termination, exists in each single genetic system, and sequential accumulation is formed under the control of multiple factors in the areas where multiple genetic systems are superimposed. Fifth, the multi-system superimposed area is rich in large gas fields and the multi-stress hinge zone in the central area is the natural gas convergence zone. Finally, the future orientation and targets of natural gas exploration in China were pointed out. First, the system A includes Sinian–Lower Paleozoic in the Sichuan Basin, Cambrian in the Tarim Basin and Cambrian–Ordovician in the Ordos Basin. Second, the system C includes the transform zone of Kuqa thrust structure, the northwestern Sichuan Basin and the southwestern Tarim Basin. Third, the system E includes the basins in the eastern China seas. Fourth, the system F includes organic-rich shales in South China and deep coal beds in the Ordos Basin in central China. Keywords: China, Large gas field, Distribution law, New area, Cratonic rift and paleo-uplift, Thrust structure of piedmont fault depression, Anticline structure of epicontinental strike–slip fault depression, Adsorption and accumulation in nano-scale spac

Formation mechanism of tight sandstone gas in areas of low hydrocarbon generation intensity: A case study of the Upper Paleozoic in north Tianhuan depression in Ordos Basin, NW China

The Upper Paleozoic in the north part of Tianhuan depression in the Ordos Basin, NW China has lower hydrocarbon generation intensity and complex gas-water relationship, the main factors controlling the formation of tight sandstone gas and the distribution of tight sandstone gas in the low hydrocarbon generation intensity area are studied. Through two-dimensional physical simulation experiment of hydrocarbon accumulation, analysis of reservoir micro-pore-throat hydrocarbon system and dissection of typical gas reservoirs, the evaluation models of gas injection pressure, reservoir physical property, and gas generation threshold were established to determine the features of tight gas reservoirs in low hydrocarbon intensity area: (1) at the burial depth of less than 3 000 m, the hydrocarbon generation intensity of (7−10)×108 m3/km2 is high enough to maintain effective charging; (2) tight sandstone in large scale occurrence is conducive to accumulation of tight gas; (3) differences in reservoir physical property control the distribution of gas pool, for the channel sandstone reservoirs, ones with better physical properties generally concentrate in the middle of sandstone zone and local structural highs; ones with poor physical properties have low gas content generally. Based on the dissection of the gas reservoir in the north Tianhuan depression, the formation of tight gas reservoirs in low hydrocarbon generating intensity area are characterized by “long term continuous charging under hydrocarbon generation pressure, gas accumulation in large scale tight sandstone, pool control by difference in reservoir physical property, and local sweet spot”, and the tight gas pools are distributed in discontinuous “sheets” on the plane. This understanding has been proved by expanding exploration of tight sandstone gas in the north Tianhuan depression. Key words: Ordos Basin, Tianhuan depression, hydrocarbon generation intensity, tight sandstone gas, reservoir-forming mechanism, distribution patter