Study On The Reservoir Mechanism Of Cambrian Zhangxia And Sanshanzi Formation,Ordos Basin

Based on the previous research achievements,drilling and seismic exploration results,combined with field geological survey data and measured fine stratigraphic section,the Lower Paleozoic Cambrian carbonate rocks in the Ordos Basin,especially Upper Cambrian Sanshanzi Formation and the Middle Cambrian Zhangxia Formation were studied.Based on the theory of sedimentology,lithofacies palaeogeography,geochemistry and reservoir geology,by applying the method of point,line and surface combination,field and indoor combination,macroscopic and microscopic combination as well as routine study and advanced test analysis combination,aimed at the Cambrian strata in the Ordos Basin,the sedimentary facies,sedimentary environment,sedimentary evolution,reservoir rock types,reservoir spaces,reservoir properties and geochemical characteristics as well as diagenetic types and theirimpact on reservoirs were comprehensively studied and the reservoir genesis and its main controlling factors were systematically analyzed.The main understandings and achievements obtained are as follows:(1)The sedimentary system of the Middle-Upper Cambrian strata in the Ordos Basin can be divided into carbonate platform,platform margin,and slope-basin sedimentary system,among which,carbonate platform sedimentary system can befurther divided into platform tidal flat,restricted platform and open platform facies,platform margin sedimentary system can be further divided into platform marginal shoal,platform marginal organic reef and platform marginal slope facies,and slope-basin sedimentary system can be further divided into upper slope,lower slope,basin margin and deep-sea plain facies.(2)Lithofacies paleogeographic maps of the two formaitons are prepared and five geographical units are identified: denudation of the ancient land facies,platform tidal flat facies,restricted-open platform facies,foreslope of platform facies,continental shelf-abyssal basin facies.Depositional models and evolution processes are established.(3)Based on reservoir petrology the Cambrian carbonate reservoir rocks in the Ordos Basin are dominated by crystalline dolomites.The reservoir space includes intercrystalline pore,dissolved pore,dissolved caves,intergranular pore and fracture.(4)Dolomitization mechansims of the study area are different between Zhangxia Formation and Sanshanzi Formation based on the analysis of different REE distribution pattern.?13CPDB of dolomite in Sanshanzi Formation is similar to seawater,while ?18OPDB varies within a narrow range of <5‰.It indicates the dolomitization fluid in Sanshanzi Formation is dominated by seawater and its dolomitizaton mechanism is seepage reflux dolomitization.(5)Carbonate reservoirs are affected by sedimentary facies,dissolution,dolomitization and burial diagenesis and making the reservoir densification after the burial period.The densification process can be summarized as follows: early diagenetic compaction ? early silicification / recrystallization(little change in porosity)? uplift dissolution / filling(or continuous deep burial)?(re-burial)Pressure-solution / recrystallization(hydrocarbon charging)? deep burial dissolution?filling and recrystallization ?tectonic uplifting fracturing / brecciation(or continuous burial)?freshwater leaching / silicification / de-dolomitization(or continuous burial)?filling.