Study On Geological Structure And Reservoir Formation Rules Of Overlap-denudation Zone In Southern Of Dongying Sag

Stratigraphic reservoirs,an important reservoir type in a petroliferous basin,mainly occur in the overlap-denudation zones between basin slopes and surrounding uplifts.The overlap-denudation zone in the southern Dongying sag lies in the southeast of the Jiyang Depression,the Bohai Bay Basin,east China.It is surrounded by Huagou-Jinjia in the west,Caoqiao-Bamianhe in the east,and the Luxi Uplift in the south.It is connected with the Boxing and Niuzhuang sub-sags through the south slope.The prospect area is about1800 km².The study targets stratigraphic traps in the overlap-denudation zone,the southern Dongying sag.Data used include seismic,logging,oil testing,and lab data.Based on structural geology,petroleum geology,and sequence stratigraphy,I use stratigraphic subdivision and correlation technique for residual formations in the overlap-denudation zone to establish the series of strata,distribution,and patterns of stratigraphic unconformities.After the evolution analysis of major faults which controlled hydrocarbon migration in this zone,I use fault closed index,a parameter calculated using SGR,faulting stress,and compressive strength of the fracture zone,to assess fault sealing properties.The preferential migration pathways are diagnosed based on fault sealing properties,crude oil properties,formation water salinity,fluid potential,and formation pressure.Finally,I evaluate stratigraphic traps preservation,establish hydrocarbon accumulation model,and predict promising prospects with hydrocarbon accumulation.Research findings are detailed as follows.In view of stratigraphic break in the overlap-denudation zone caused by multi-phase tectonic activities and water oscillation,the key to the study of unconformity traps is to identify unconformity interfaces and stratigraphic subdivision and correlation of residual formations.Four log curves,i.e.GR,SP,AC,and COND which are sensitive to lithology,petrophysical properties,and fluid properties,are reconstructed and split to generate a composite curve for single-well unconformity identification,the minimum point on which corresponds to the abrupt contact,i.e.unconformity interface.Stratigraphic correlation of unconformity interfaces and residual formations are conducted using time-frequency sections because on higher-order spectrum which reflects stratigraphic cycle,the time-frequency discontinuity point corresponds to the abrupt interface.Based on single-well and cross-well examination for the identification of unconformity interfaces,the series of strata,grade,type,and tectonic style of unconformity are established,together with two stratal configuration models and eight assemblages on sections.Oil and gas in the region of interest originated in the Boxing and Niuzhuang sub-sags and migrated along Boxing,Shicun,and Wangjiagang source faults into reservoir beds.Fault sealing property varies spatially and is dependent on fault activity rate,lithologic contact,faulting stress,and compressive strength of the fracture zone.To quantitatively estimate fault sealing property,a parameter,fault closed index?FCI?,is presented to be the ratio of faulting stress to compressive strength.The Boxing fault is a grade-2 fault which was active in a long period of time in the southern Boxing sub-sag.As per the index calculated,its western section is poorly sealed,and its middle and eastern section is well sealed in the deep zone.In the middle and shallow zone,its middle section is poorly sealed,and its western and eastern sections are well sealed.In other words,oil and gas tend to migrate along the deep section of the fault in the west and middle and shallow section in the center.The quantitative examination of fault sealing property avails the prediction of hydrocarbon accumulation in the overlap-denudation zone.Crude oil in the overlap-denudation zone,the southern Dongying sag is of high gravity.In the lateral direction,oil density increases with the distance of migration.Crude oil in Jinjia close to the source sub-sag has smaller density and viscosity than that in Caoqiao.Deep formation water is of CaCl₂ type,and middle and shallow formation water is of NaHCO₃ type.The salinity increases with buried depth;thus,formation water in the sub-sags has higher salinity than that in the overlap-denudation zone.Fluid potential exhibits a ring-shaped distribution around the south slope of the Dongying sag.Fluid potential is high in the sub-sags and low in Jinjia and Caoqiao where oil and gas accumulate.Abnormal pressure in the source sub-sags was favorable for hydrocarbon expulsion and migration.The overlap-denudation zone with normal pressure was the destination of hydrocarbon migration.In accordance with above four controls,there may be three migration pathways with sufficient dynamic force,i.e.Fanjia-Zhenglizhuang-western Jinjia in the Boxing sub-sag,Boxing-Caonan,and Niuzhuang-Wangjiagang-Caobei,in the overlap-denudation zone.There are three models,i.e.?1?fault-sands-fault-unconformity barrier,?2?fault-sands-unconformity barrier,and?3?fault-sands-fault-unconformity-caprock barrier,to describe hydrocarbon accumulation in the overlap-denudation zone,the southern Dongying sag.Trap efficiency along the preferential migration pathway is dependent on barrier quality;weathered clay bed,mudstone,and local igneous rocks are the most favorable barriers.Oil column height is dependent on the breakthrough pressure of barrier.Barrier quality is predicted using log-constrained seismic inversion based on the relationship between breakthrough pressure and GR and AC logs.Exploration targets are then established based on preferential migration pathway,favorable trap,and efficient barrier.Using above techniques,13 exploratory wells were deployed in 2014 for the exploration of the overlap-denudation zone,the southern Dongying sag.Oil reservoirs were drilled in 8 wells,accounting for 73%of 11 wells finished.Cumulative probable reserves and prognostic reserves were booked to be 1134×10⁴ t and 2057×10⁴ t,respectively.