Formation mechanism of fault accommodation zones under combined stress in graben basin:Implications from geomechanical modeling

A fault accommodation zone is a type of structure that is defined as regulating displacement and strain between faults structure.Increasing numbers of fault accommodation zones are being identified in graben basins,indicating the potential exploration target and petroleum accumulation areas.This study aims to analyze the formation mechanism and development of fault accommodation zones under combined stress by a numerical simulation method considering geomechanical modeling.Using three-dimensional(3-D)seismic interpretation and fractal dimension method,exampled with the Dongxin fault zone,the fault activity and fault combination pattern were conducted to quantitatively characterize the activity difference in fault accommodation zones.Combined with mechanical experiment test,a geomehcanical model was established for fault accommodation zones in a graben basin.Integrating the paleostress numerical simulations and structural physical simulation experiment,the developmental characteristics and genetic mechanism of fault accommodation zones were summarized.Influenced by multi movements and combined stresses,three significant tectonic evolution stages of the Dongxing Fault Zone(DXFZ)were distinguished:During the E_(s)^(3)sedimentary period,the large difference in the stress,strain,and rupture distribution in various faults were significant,and this stage was the key generation period for the prototype of the DXFZ,including the FAZ between large-scale faults.During the E_(s)^(2)sedimentary period,the EW-trending symmetric with opposite dipping normal faults and the NE-SW trending faults with large scale were furtherly developed.The junction area of two secondary normal faults were prone to be ruptured,performing significant period for inheriting and developing characteristics of fault accommodation zones.During the Es1 sedimentary period,the high-order faults in the DXFZ exhibited the obvious fault depressions and strike-slip activity,and the fault accommodation zones were furtherly inherited and developed.This stage was the molded and formative period of the FAZ,the low-order faults,and the depression in the DXFZ.

Prediction of Shale Gas Pressure based on Multi-channel Seismic Inversion in Fuling

The accurate prediction of formation pressure is important in oil/gas exploration and development.However,the achievement of this goal remains challenging,due to insufficient logging data and the low predictive data accuracy from seismic data.In this work,a case study was carried out in the Baima area of Wulong,in order to develop a workflow for accurately predicting shale gas formation pressure.The multi-channel stack method was first used,as well as the inversion of single-channel seismic data,to construct velocity and density models of the formation.Combined with the existing welllogging data,the velocity and density models of the whole well section were established.The shale gas formation pressure was then estimated using the Eaton method.The results show that the multi-channel seismic stacking method has a higher accuracy than the inversion of the formation velocity obtained by the single-channel seismic method.The discrepancies between our predicted formation pressure and the actual formation pressure measurement are within an acceptable range,indicating that our workflow is effective.

Reservoir formation conditions and key exploration & development technoloiges in the Yingdong Oilfield in western Qaidam Basin

Located in east part of Yingxiongling structural belt in the Qaidam Basin,the Yingdong oilfield has a extremely complicated ground condition.Due to no significant discovery,this oilfield was considered to have no favorable geologic conditions for formation of oil or gas reservoir.In the past few years,with continuous improvement in the mountain 3D seismic surveys and logging data interpretation,some breakthroughs were obtained in 2010,and the Yingdong oilfield,the largest-scale reserves of a single reservoir with highest organic matter abundance,most favorable physical property and optimal development efficiencies in the Qaidam Basin,had been discovered,the production capacity was up to 0.55×10^(6) t.Through detailed analyses of the Yingdong oilfield,some studies,such as hydrocarbon accumulation conditions and technical challenges,are carried out,and following conclusions can be achieved.The Yingxiongling area is located in Mangya hydrocarbon-generation sag in the west part of the Qaidam Basin,its oil sources are rich;the Neogene Xiayoushashan Formation and Shangyoushashan Formation are dominated by wide and gentle delta frontdshore-shallow lacustrine sediments with interbeds of sandstone and mudstone,the sandbodies are widely distributed with favorable physical condition,and the mudstone is the key caprock,combined with high-quality Paleogene hydrocarbon source rocks,a complete source-reservoir-cap assemblage can be formed.Large-scale detachment faults of the Yingdong area connect high-quality Paleogene hydrocarbon source rocks with middle-shallow buried structural traps,thus,reservoirs formed in the early stage are modified,and at the same time,hydrocarbons formed in the later stage continue to migrate and accumulate;in this way,the deep and shallow faults form a relay-style hydrocarbon transport system,and hydrocarbons are accumulated in the shallow structural traps in the later stage;in this area,the middle-shallow faults have good lateral plugging performance which is favorable for preservation of oil and gas.For complex landforms and reservoir features in the Yingdong area,the integral 3D seismic acquisition,processing and interpretation technology is developed for complex mountain areas to provide a reliable foundation for hydrocarbon exploration.For some problems in the Yingdong oilfield like long oil/gas-bearing intervals,great diffi-culty in identification of fluids,the development mode of multiple oil/gas/water systems in the long intervals is established,and the geologic modeling technology with constraint of multiple conditions on complex fault blocks is also developed.Thus,hydrocarbon accumulation mechanism in the Yingdong oilfield is clear,and some complex key technology of engineering are well solved,providing necessary geologic theories and technical supports for high-efficiency development and rapid production construction in the Yingdong oilfield.