Study On The Dissolution And Characteristics Of Lower Paleozoic Carbonate Reservoir In Nanpu Depression

Carbonate reservoir is one of the important petroleum-prospecting areas in Nanpu depression.Dissolution plays a vital role in the development of carbonate reservoir space and hydrocarbon migration and accumulation.Taking the Lower Paleozoic carbonates in Nanpu depression and surrounding areas as research object,comprehensive studies on rock types,dissolution rules,dissolution characters and development stages,dissolution development modes of the Lower Paleozoic carbonate reservoir have been conducted based on the analysis of outcrop,core,slice,as well as well logging,by the means of experimental simulation,and under the guidance of petrology,reservoir geology,structural geology,geomorphology and buried hill accumulation theory.At the same time,the hydrocarbon accumulation models have been constructed combining with the petroleum geological conditions.The rock types of carbonate reservoir in the Lower Paleozoic strata in Nanpu depression can be divided into two kinds: limestone and dolomite,and limestone is dominated.Dissolution rules of carbonate rocks have been identified by experimental simulation,and the result indicates that the solubility of calcite is increased with additional small amount of dolomite in limestone in supergene condition,and the critical point of dolomite content is 13%.Over this point,the dissolution of carbonate rocks will decrease with increasing of dolomite content,and overall limestone is easier to dissolve than dolomite.In burial condition,the dissolution extent of carbonate rocks increases with the increase of temperature and pressure,and dolomite is easier to dissolve than limestone.Texture-and composition-depended selective dissolution is the main mechanism of carbonate dissolution.Calcite and dolomite both have characteristic dissolution patterns,which are the “corroded cone” and “honeycomb pore”,respectively.For the signs of carbonate dissolution,hypergene and buried dissolution have been distinguished in aspect of outcrop core,slice,and logging.The stage of dissolution development in supergene condition all has been classified into two periods according to the structural evolution history.The stage of buried dissolution has also been classified into two periods according to inclusion analysis.They are from the late Dongying to the early Guantao and the middle Minghuazhen.Furthermore,the dissolution development modes have been built combining with the development characteristics and major controlling factors of dissolution.On this basis,hydrocarbon source and hydrocarbon accumulation conditions in carbonate reservoir are identified from the analysis of basic geology conditions of Nanpu depression,such as characteristics of source rock,reservoir and migration system.Two kinds of hydrocarbon accumulation models of the Lower Paleozoic carbonate reservoir have been established according to the location relationship of source rock and reservoir,which are source rock in contact with reservoir-direct charging model and source rock separated from reservoir-lateral migration model.