The Research On Genesis Mechanism Of High Quality Lacustrine Carbonate Reservoir In Jiyang Depression

In recent years,lacustrine carbonate reservoirs as unconventional reservoirs has become the exploratory highlight,but the unknown of formation mechanism of lacustrine carbonate high quality reservoirs constraint the exploration activity.This thesis systematically studied lacustrine carbonate rocks of the upper fourth member of the Eocene Shahejie Formation(Es4s)around Sikou Sag and Dongying depression,Jiyang Sub-basin.Spatial distribution of lacustrine carbonate rock of Es4 s in paleogene in Jiyang sub-basin has significant relationship with surrounding paleozoic bedrock.The ?13C and ?18O value of Sikou carbonates differ significantly from Eocene marine values,and the 87Sr/86 Sr ratio of carbonate minerals(0.710210 to 0.710844)are higher than Eocene marine and close to surrounding Palaeozoic carbonate bedrock indicate its lacustrine originHigh quality reservoirs of lacustrine carbonate rock in Jiyang sub-basin consist of reef-shoal type reservoir,karst reservoir and lamellar dolomite reservoir,developed seven types of porosity.Horizontal distribution of physical property of reservoirs is controlled by the distribution of lithofacies,whereas the unconformity and high frequency sedimentary cycles control the vertical distribution.Diagenetic environment is classified into four categories: saline lake water,meteoric water,surface environment and buried environment.Diagenetic processes improving reservoir quality include dissolution,dolomitization and fracture development.Reservoir quality was decreased by cementation and compaction.The characteristic and distribution of shoal type reservoir in ramp zone were chotrolled by sedimentary process and diagenetic environment.The dolotimized bioclastic and oolitic grainstones of the inshore zone which underwent strong meteoric dissolution and slight compaction,containing both primary intergranular and intragranular dissolution porosity,the bulk carbonate have?13C values between 0.84 and 3.03 PDB and?18O values from-10.52 to-2.08 PDB;Due to lack of meteoric affection and intense shallow burial cementation and compaction,the dolotimized intraclastic grainstone in off-shore zone just develop intergranular dissolution porosity related to the organic acid,the bulk carbonate have ?13C values between 2.79 and 5.02 PDB and ?18O values from-6.6 to-3.57 PDB.The geochemical data show that inshore zone was affected by much more meteoric fluid than nearshore zone.The dolotimized mudstones in the off-shore zone develop intercrystalline porosity and fractures because of the dolomitization and fracturing.The affection of decreased meteoric fluid and enhanced basinal fluid along a transection from shoreline to deep lake resulted in the porosity changing from normal porosity to nanometer porosity and property and the decreased property.The formation of lacustrine carbonate karst-type reservoir is controlled by corrosion in supergeneperiod,epogenenesis and burial period.Fracture and geode are the main pore types of karst-type reservoir in supergeneperiod.Its development scale is controlled by the original sedimentary thickness.The karst-type reservoir of epogenenesis mainly develops large scale karst cave.The single karst cave consists of top vertical corrosion zone,filling karst breccias zone and bottom weak corrosion zone.The unfilled top vertical corrosion zone and filling karst breccias zone are good reservoir space.The pore of buried karst reservoir presents to honeycomb-type or pinhole-type and is usually filled with brown oil and black asphalt.The katst reservoir of supergeneperiod mainly develops in shore onlap zone of transgressive system tract and the top of progradational-type shoal formed during the water withdralwal period in highstand system track.The katst reservoir of epogenenesis mainly distributes under the unconformity.The distribution of the katst reservoir in burial period is controlled by the effective migration path of the acidic fluid.These lacustrine carbonate deposits consist of abundant very finely to finely crystalline dolomite that occurs in all grain-and mud-dominated sediments.The dolomite distribution has obvious cyclicity in the vertical,and the content of dolomite is increased towards the center of the basin at the same deposition period.Dolomite crystal are 3~10 ?m in length,subhedral to anhedral in shape,are associate with clay?gypsum and pyrite,and typically replaced the matrix,but also occur as thin lamellae within ooid cortices and isopachous cement.textural relations indicated that the dolomite predates all other diagenetic phases,including equant sparry calcite cement,anhydrite and coarsw calcite cement.The 87Sr/86 Sr ratios of pan carbonate minerals are obvious higher than modern sea water and same with the surrounding Palaeozoic bedrock indicate a predominantly terrigenous source for Mg2+ derived from the chemical weathering of bedrock.The high Mg/Ca ratio lacustrine water has result in the formation of dolimite,due to the formation of gypsum and salt under the strong evaporate condition.Most of the dolomite formation by the replacement of precursor,whereas the lamellae dolomite and dolomite cement may direct precipitation of dolomite from lakewater.Depleted ?18O values,increased degree of order and the petrology features suggest the early-formed dolomite was variably recrystallized during burial.