The Integrity Of Caprocks And Gas Accumulation In Yingqiong Basin

Yingqiong Basin is the important Cenozoic petroliferous basin in China South Sea.After several years' petroleum exploration,it has been found many large oilfields and petroleum bearing structures such as DF-A,DF-B and YC-B.The formation tests and pressure prediction results show that the ground under 2800-3000 m generally develop abnormal high pressure,and the maximum stratigraphic pressure coefficient can reach 2.3.With the gradual improvement of the petroleum exploration and development in shallow layers,middle and deep overpressured layers become the main target of the natural gas exploration.Whether the caprocks can effectively seal the oil and gas turns to be a crucial questions for the research.Taking the diapir area in central depression and Ledong-Lingshui depression as examples to describe the development characteristics and the sealing modes of the caprocks in different accumulation systems.Based on the triaxial compression experiment of caprocks rock mechanics,by using the Mohr-Coulomb criterion,Byerlee friction law,Goetze criterion method and the principle of energy conservation after peak value to qualitatively evaluate brittle-ductile stage of different caprocks,and then to analyze the failure factors of the caprocks integrity in different accumulation systems.The main recognition obtained is as follows:There are three accumulation systems in Yingqiong Basin that have developed six sets caprocks: Ledong formation,the first member of Yinggehai formation,the second member of Yinggehai formation-the first member of Huangliu,the second member of Meishan formation,the first member of Sanya formation and the second member of Lingshui formation caprocks.The caprocks in Huangliu and Yinggehai formation are all the regional caprocks with the most extensive distribution and the most stable deposition in the whole basin,and the other four sets of caprocks are the local caprocks.Through detailed dissection of typical gas reservoirs,it categorizes the three accumulation systems into four sealed modes: fault with direct caprock sealed mode in ultra shallow accumulation system;diaper with direct caprock sealed mode in shallow diapir accumulation system;middle-deep accumulation system is further divided into regional caprock sealed mode and fault with partial caprock sealed mode.The caprocks of Yingqiong basin are mainly argillaceous rocks,the sealing mechanism of caprocks is capillary sealing,which is reflected by the replacement pressure.The results show that the microcosmic sealing ability of the caprocks of Yingqiong basin is generally good,and whether the integrity of caprocks is damaged becomes the key of gas accumulation.With the change of buried depth,the caprocks show different brittle and ductile characteristics,in different brittle-ductile stage,the deformation mechanism and modes are different.It was determined that the critical condition from brittleness to brittle-ductile transition in the first section of Huangliu formation is 32 MPa,the depth is 1390 m,and the critical condition from brittle-ductile transition to plasticity is 123 MPa,about 5347 m.The critical condition from brittleness to brittle-ductile transition in the first section of the Sanya formation is 78 MPa,the depth is about 3390 m,and the critical condition from brittle-ductile transition to plasticity is 219 MPa,about 9522 m.Therefore,ultra shallow and shallow accumulation systems have develop the brittle caprocks,faults,subseismic faults and structural fractures are the failure conditions of integrity;while in middle-deep accumulation system,the caprocks are in the brittle-ductile stage and their integrity are controlled by the hydraulic fracturing and faulting.Basing on the identifying caprock sealing modes and major factors of caprock integrity in different accumulation systems,using the different evaluation methods to respectively quantitatively assess destruction of caprock integrity from subseismic faults,structural fractures,faults and hydraulic fractures.In the ultra shallow and shallow accumulation systems,the connectivity is the fundamental reason that subseismic faults and tectonic fractures can break the cap and cause leakage,including the connectivity between subseismic fault with sandstones and the interconnection of the structural fractures.The results show that when the mudstone thickness comes over 10 m,subseismic fault can not connect with reservoirs but the fractures still can form connectivity to leak the gas,while the thickness grows beyond 60 m,no matter of the subseismic faults or structural fractures can not destruct the integrity of caprock.Faults in different brittle-ductile caprocks have different deformation mechanisms,and their quantitative evaluation methods are various.Therefore,using juxtaposition thickness to characterize vertical leaking of the faults in brittle caprocks,Statistics show the critical juxtaposition thickness is 86-131 m,when juxtaposition thickness is less than critical scales,the brittle caprock integrity will be destroyed;faults in brittle-plastic caprocks deform to form smearing,and the caprock integrity critical SSF values are 1.74-2.56.Middle-deep level accumulation system has developed a large amount of lithologic gas reservoir,caprock integrity is less influenced by faults,subseismic faults and structural fractures.Hydraulic fractures caused by overpressure are the main reason of oil-gas leaking.Combined single well leakage test,logging materials,and rock mechanics parameters to calculate the caprock hydraulic fracturing pressure index,and make quantitative evaluations of current and historical time hydraulic fracturing of caprocks in research area.Basing on the evaluation results,it has four hydraulic fracturing dynamic transformation modes:(1)sealing mode;(2)fracturing into sealing mode;(3)sealing into fracturing mode;(4)fracturing mode.Basing on the results of quantitative evaluation,combined with experimental methods such as fluid inclusion and analysis and quantitative fluorescence technology,to restore the history of gas migration,and finally determine the caprocks integrity in controlling gas accumulation and vertical distribution.