Evolution of the CBM reservoir-forming dynamic system with mixed secondary biogenic and thermogenic gases in the Huainan Coalfield,China

The secondary biogenic gas is an important original type of the coalbed methane (CBM) in China. Based on the analyses of sedimentary and burial history of the Permian coal-bearing strata, combined with thermal history and gas generation process of coals, the CBM reservoir-forming dynamic system with mixed secondary biogenic and thermogenic gases in the Huainan Coalfield is subdivided into four evolutionary stages as follows: (i) shallowly-buried peat and early biogenic gas stage; (ii) deeply buried coal seams and thermogenic gas stage; (iii) exhumation of coal-bearing strata and adsorbed gas lost stage; and (iv) re-buried coal-bearing strata and secondary biogenic gas supplement stage. The Huainan CBM reservoir-forming model has the features of the basin-centered gas accumulation. The evolution of the reservoir-forming dynamic system proves that the thermogenic gas is not the main gas source for the Huainan CBM reservoir. Only the secondary biogenic gases as an additional source replenish into the coal bed after basin-uplift, erosional unroofing and subsequent scattering of thermogenic gases. Then this kind of mixed CBM reservoirs can be formed under suitable conditions.

Evolution characteristics and exploration targets of Permian clastic rock reservoirs in Bohai Bay Basin,East China

Based on core observation, thin section examination, fluid inclusions analysis, carbon and oxygen isotopic composition analysis, and other approaches, the structural and burial evolution histories were investigated, and the diagenetic evolution process and genetic/development models were systematically discussed of the Upper Paleozoic Permian clastic rock reservoirs in the Bohai Bay Basin, East China. The Bohai Bay Basin underwent three stages of burial and two stages of uplifting in the Upper Paleozoic. Consequently, three stages of acid dissolution generated by the thermal evolution of kerogen, and two stages of meteoric freshwater leaching occurred. Dissolution in deeply buried, nearly closed diagenetic system was associated with the precipitation of authigenic clay and quartz, leading to a limited increase in storage space. Different structural uplifting–subsidence processes of tectonic zones resulted in varying diagenetic–reservoir-forming processes of the Permian clastic reservoirs. Three genetic models of reservoirs are recognized. The Model I reservoirs with pores formed in shallow strata and buried in shallow to medium strata underwent two stages of exposure to long-term open environment and two stages of meteoric freshwater leaching to enhance pores near the surface, and were shallowly buried in the late stage, exhibiting the dominance of secondary pores and the best physical properties. The Model Ⅱ reservoirs with pores formed in shallow strata and preserved due to modification after deep burial experienced an early exposure-open to late burial-closed environment, where pore types were modified due to dissolution, exhibiting the dominance of numerous secondary solution pores in feldspar and the physical properties inferior to Model I. The Model Ⅲ reservoirs with pores formed after being regulated after multiple periods of burial and dissolution experienced a dissolution of acidic fluids of organic origin under a near-closed to closed environment, exhibiting the dominance of intercrystalline micropores in kaolinite and the poorest physical properties. The target reservoirs lied in the waterflood area in the geological period of meteoric freshwater leaching, and are now the Model Ⅱ deep reservoirs in the slope zone–depression zone. They are determined as favorable options for subsequent exploration.

Vertical migration through faults and hydrocarbon accumulation patterns in deepwater areas of the Qiongdongnan Basin

In the Qiongdongnan Basin, faults are well developed.Based on the drilling results, the traps controlled two or more faults are oil-rich. However, when only one fault cut through the sand body, there is no sign for hy-drocarbon accumulation in the sandstone. In terms of this phenomenon, the principle of reservoir-forming controlled by fault terrace is proposed, i.e., when the single fault activates, because of the incompressibility of pore water, the resistance of pore and the direction of buoyancy, it is impossible for hydrocarbon to ac-cumulate in sandstone. But when there are two or more faults, one of the faults acts as the spillway so the hydrocarbon could fill in the pore of sandstone through other faults. In total five gas bearing structures and four failure traps are considered, as examples to demonstrate our findings. According to this theory, it is well-advised that south steep slope zone of Baodao-Changchang Depression, south gentle slope zone of Lingshui Depression, north steep slope zone of Lingshui Depression, and north steep slope zone of Baodao Depression are the most favorable step-fault zones, which are the main exploration direction in next stage.

Orderliness of Hydrocarbon Accumulation Distribution in Rift Basins of Eastern China

The Cenozoic rift basins in eastern China show a clear temporal and spatial zonation and episodic tectonic evolution, which control their episodic hydrocarbon generation and zonal accumulation. In this paper, based on the study of depositional architecture, hydrocarbon migration system and dynamic evolution in the rift basins, combinations of hydrocarbon accumulation elements were analyzed using sequence stratigraphy. Hydrocarbon distribution in system tracts with different sequence orders was further studied. And we summarized stacking patterns and horizontal combination relationships for different types of reservoirs, such as lithological, tectonic-lithological, tectonic and stratigraphical reservoirs which can be observed from depression center to basin margin. The result reveals that various scales of pools exhibit significant distribution and evolution orderliness in different pool-forming units, i.e., depositional systems, plays and depressions. The regular distribution of various scales of pools is closely related to tectonic evolution and depositional filling in the rift basins. The result can be applied to the fine petroleum exploration in rift basins in eastern China. It will promote the scientific prediction and evaluation of reservoir types and their spatial distribution, lead to the active shifts of exploration targets in different zones, and thus support the stable progress of fine exploration in mature exploration areas.

Study on fault-controlled hydrocarbon migration and accumulation process and models in Zhu I Depression

Through the analysis of the faults and their internal structure in Zhu I Depression,it is found that the internal structure of the late fault is obviously segmented vertically.It develops unitary structure(simple fault plane)in shallow layers,binary structure(induced fracture zone in hanging wall and sliding fracture zone in footwall)in middle,layers and ternary structure(induced fracture zone in hanging wall and sliding fracture zone in middle,and induced fracture zone in footwall)in deep layers.Because the induced fracture zone is a high porosity and permeability zone,and the sliding fracture zone is a low porosity and ultra-low permeability zone,the late fault in middle layers has the character of"transporting while sealing".The late fault can transport hydrocarbon by its induced fracture zone in the side of the hanging wall and seal hydrocarbon by its sliding fracture zone in the side of the footwall.In deep layers,the late fault has the character of"dual-transportation",induced fracture zones in both sides of hanging wall and footwall can transport hydrocarbon.The early fault that only developed in the deep layers is presumed to be unitary structure,which plays a completely sealing role in the process of hydrocarbon migration and accumulation due to inactivity during the hydrocarbon filling period.Controlled by hydrocarbon source,early/late faults,sand bodies and traps,two reservoir-forming models of"inverted L"and"stereo-spiral"can be proposed in middle layers,while two reservoir-forming models of"cross fault"and"lateral fault sealing"are developed in the deep layers of Zhu I Depression.

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 ofis 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.

How argillaceous reservoirs exhibit better quality than silty mudstones?Anomalous behavior of shale gas-bearing properties of continental fine-grained sediments in Southwest China and its possible forcing mechanisms

Achieving a thorough understanding of how primary sedimentary granularity drives considerable heterogeneity in internal reservoir attributes of terrigenous fine-grained deposits is of great significance.We investigated the quantitative differentiation and its corresponding driving forces of physical reservoir properties and pore-structure characteristics of silty-mud sediments in the Upper Triassic Xujiahe Formation(SW China)using a multi-method approach.The results show that the micro-mesopore volume and surface area of mudstones/shales are apparently higher than those of silty mudstones and a remarkable threefold rise in average permeability also presents.Extensively distributed bitumen pores occurring mostly along brittle mineral grains or forming clay-organic complexes make considerable contributions to shrinking microcracks.Furthermore,an evidently higher concentration of clay minerals in mudstone/shale reservoirs is primarily responsible for development of the two types of clay intercrystalline pores distributed along grain aggregates and between well-oriented platelets.These two major causes facilitate the formation of micro-bedding fractures/non-bedding micro fractures and connected fracture and pore-fracture networks,and also high-quality argillaceous reservoirs by strongly enhancing storage spaces and seepage capacities.Finally,a conceptual model is established for interpreting a differential reservoir-forming mechanism and corresponding two-sided effects on petrophysical and reservoir quality properties for continental silty-mud sediments.

Types of biogenic quartz and its coupling storage mechanism in organic-rich shales: A case study of the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation in the Sichuan Basin, SW China

Biogenic quartz in the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation(WufengLongmaxi)shale layers in the Sichuan Basin and its periphery is qualitatively analyzed and quantitatively characterized by organic petrologic,mineralogic,and geochemical methods to find out the coupling effect between organic matter and quartz.(1)There are two types of biogenic quartz in the shale layers:Type I quartz is submicron quartz appearing in clusters around the organic matter.Type II quartz is in nano-scale grain size and floats in spherical shape on organic matter,with grains in point-to-point or surface-to-surface contact;this type of quartz is mainly biologic origin and slightly affected by hydrothermal activity in local parts.(2)The reservoirs in the Wufeng-Longmaxi formations is consistent in distribution with biogenic silica content in them,and mainly concentrated at the bottom of the Wufeng-Longmaxi formations,and is thinner in the Changning and Weiyuan regions,while thicker in the Fuling region.(3)The biogenic quartz in the Wufeng-Longmaxi shale worked through the entire evolution process of hydrocarbon generation.The presence of biogenic quartz can enhance the development of organic matter pores and microcracks,and can effectively preserve the organic matter pores and residual intergranular pores,forming"biological silicon intergranular pores,organic pores and micro-fractures".This would benefit later hydraulic fracturing and result in high production/stable production of well.The coupling effect between biogenic quartz development and organic matter evolution and hydrocarbon generation is a critical factor for high-quality shale reservoir development.

Geologic characteristics, controlling factors and hydrocarbon accumulation mechanisms of China’s Large Gas Provinces of low porosity and permeability

Based on the analysis of the geological characteristics and controlling factors, we analyzed the formation mechanism of different types of gas reservoirs. The main characteristics of gas provinces with low porosity and permeability are mainly as follows: large area, low abundance, small gas pools and large gas provinces; widely distributed excellent hydrocarbon source rocks with closely contacted source-reservoir-cap association; development mainly in large continental depressions or in paralic shallow-river delta systems; many kinds of traps coexisting in large areas, dominantly para-layered lithologic, digenetic and capillary pressure traps; double fluid flow mechanisms of Darcy flow and non-Darcy flow; complicated gas and water relations; and having the resource distribution of highly productive "sweet spots", banding concentration, and macroscopically large areas integrated. The main controlling factors of large sandstone gas provinces with low porosity and permeability are stable dynamic backgrounds and gentle structural frameworks which control the extensive distribution of alternate (interbedded) sandstones and mudstones; weak hydropower of large gentle lake basins controlling the formation of discontinuous, low porosity and permeability reservoirs in shallow-water deltas; regionally differential diagenesis and no homogeneous digenetic facies controlling the development of favorable reservoirs and digenetic traps; and weak and dispersive reservoir-forming dynamic forces leading to the widely distributed small traps with low abundance. Low porosity and permeability gas provinces with different trap types have different formation mechanisms which include fluid diversion pressure difference interactive mechanism of lithologic-trap gas accumulations, separated differential collection mechanism of digenetic-trap gas accumulations, and the Non-Darcy flow mechanism of capillary-pressure gas accumulations.

Accumulation conditions and key exploration&development technologies of heavy oil in Huanxiling oilfield in Liaohe depression,Bohai Bay Basin

The Huanxiling oilfield is located in the southern part of the western slope of the western sag in Liaohe depression.The west side of this oilfield is connected with two sets of high-quality source rocks of Member 3 and Member 4 of Shahejie Formation in Qingshui sub-sag.The oilfield has fan delta,turbidite fan and other types of reservoirs,it also has cap rock of thick mudstone in Member 3 and Member 4 of Shahejie Formation.Under background of the warped basement,the warped fault-block draped compound trap zone are developed,which includes nine types of trap.From perspective of hydrocarbon accumulation,the slope of this area has always been the target area for hydrocarbon migration and accumulation.Inclusion analysis shows that there are multiple stages of hydrocarbon charging in this area,and the main reservoir forming period is the sedimentary period of Member 3 of Shahejie Formation and the sedimentary period of Dongying Formation.High-quality source-reservoir-cap conditions ensure large-scale hydrocarbon accumulation in this area.Based on the theory of compound hydrocarbon accumulation,many types of oil and gas reservoirs,including light oil reservoir and heavy oil reservoir,have been found in this area,with total reserves of 500 million tons.In view of the oilfield characterized by large reservoir burial span,multiple oil-bearing strata,strong heterogeneity and various types of oils,multi-batch seismic data processing&interpretation technology and thin reservoir inversion technology based on geological model are established in the preliminary exploration period,steam-flooding physical simulation technology of heavy oil,oil-reservoir fine description technology of thermal recovery heavy oil,steam huff and puff technology of ordinary heavy oil and steam-flooding technology of mid-deep buried heavy oil are developed in the development period,and technologies such as separate-layer injection,selective injection,sand control and lifting of heavy oil are matched and improved.These technology series provides technical guarantee for efficient exploration and development of Huanxiling oilfield.