Study On Fault Carrier Systems And Its Relations To Hydrocarbon Accumulation In Dong-pu Sag, Bohai Gulf Basin

The study on fluid carrier systems and hydrocarbon accumulation is a frontier field and hot science problem in petroleum geology presently. Dong-pu Sag is a fault basin in palaeogene, with overlying depressed-type sediments in Neogene. Since polyphase tectonic movements and long-term fault activities were experienced, faults are abundant in this sag. Hydrocarbon-generating ability in the shallow strata up the Es_l member is bad, gas and oil accumulated in the shallow reservoir is mainly from the major hydrocarbon source rocks in the deep strata of Es₃ member. Fault carrier systems are important medium for fluid migration from deep to shallow. Also they are important factors to compose of the structural traps. Therefore, it is necessary to study on the characteristic of the fault carrier system and its relations to hydrocarbon migration and accumulation to realize the accumulation rule of shallow reservoirs and meaningful for bringing a discovery of new fields in petroleum exploration in Dong-pu Sag.On the basis of sedimentary setting and tectonic evolution, the main active periods of major faults were confirmed at first, then many affecting factors were considered comprehensively, including lithological features of fracture zone, attitudes of faults and stress field etc, evaluating qualitatively to quantitatively on vertical carrier abilities of faults connecting source rock and their vertical and lateral seal abilities by calculating angle between the main stress direction and trend line of fault, migration velocity of oil and gas and the parameter SGR(the shale gouge ratio).The pattern of fault carrier systems was established upon the dynamic evolution.Furthermore, on recordings of fluid activity, more confident evidences of hydrocarbon migration along fault zone was provided by the key techniques including carbon and oxygen isotopes analyzing of calcite vein in fault belt, homogenization temperatures of secondary fluid inclusions and GOI(grains with oil inclusion). Accumulation timings and periods of hydrocarbon were studied by combining the results of K-Ar dating of authigenic illites,diagenesis and homogenization temperature of secondary fluid inclusions.Finally,the mode of hydrocarbon accumulation evolution in fault carrier systems was established, and the relationships between the fault carrier systems and hydrocarbon accumulation were discussed.The main developments including: Firstly, Activity of shallow faults were weaker since deposition of Dongying Fm (Ed), with the nature of open and seal intermittently influenced by tectonic movement of two periods, there were two major open periods of faults at the end of deposition of Dongying Fm and Minghuazhen Fm. Secondly, there were three style of transporting patterns of fault carrier system:the highly transporting pattern in represent of Lan-Liao Fault, the formerly open and later highly transporting pattern in part layers in represent of Huanghe Fault and Changyuan Fault, and the formerly open and later fulfilled with mud and highly sealed pattern in represent of Wendong Fault and Wenxi Fault. Thirdly, faults connecting source rock were crucial to form shallow hydrocarbon reservoirs. There were two periods of charge and accumulation of hydrocarbon in Dongying Fm (Ed) and Es_l member, being consistent with the two periods of active and open nature of faults. The next active of faults lately caused re-charge and re-accumulation of hydrocarbon while the existed hydrocarbon reservoirs would be adjusted. The hydrocarbon from destroyed reservoirs and from second generation in deep source rocks would migrated to shallow strata influenced by openly again of faults, there were intensive relation between hydrocarbon accumulation or dissipation and seal ability in relative static period.