Research On The Dissolution Mechanism Of Deep Carbonate Rocks

Because continuous breakthroughs have been made in exploration of oil and gas,the study on reservoir characteristics and genetic mechanism of deep carbonate reservoir is becoming more and more important.Further research shows that the secondary dissolution pore is the main reservoir space of deep carbonate reservoir,and the strata which reaches a depth of 8000 meters is still developing karst caves,indicating that the dissolution plays an important role in the formation of reservoir space of deep carbonate reservoir.Therefore,by taking the carbonate reservoir of Cambrian in Bachu-Tabei area as an example,we carry out a systematic study on the mechanism of deep carbonate dissolution in combination with the actual geological analysis with simulation experiments.On the basis of tectonic and sedimentary characteristics,taking rock core observation and slice identification as the main means to analyze the types and characteristics of Cambrian carbonate dissolution in Bachu-Tabei area in combination with scanning electron microscope,catholuminescence,fluid inclusion,geochemical analysis and logging data.The results show that the syngeneic-penecontemporaneous atmospheric freshwater dissolution generally developed in carbonate platform margin reef-shoal,platform interior mound-shoal and gypsum-bearing dolomite flat of Cambrian in Bachu-Tabei area,they provided migration channel and storage space for diagenetic fluid in burial environment;there also developed weathering crust palekarst in Yaha-Yingmaili and south of Kuqa Depression;the organic acid dissolution and hydrothermal dissolution were developed in burial environment,but the reservoir space was remoulded mainly by organic acid fluid.Based on the analysis of actual dissolution and the research status of corrosion simulation experiment,an experiment which simulated the seepage corrosion of carbonate rock during the process of burial to analyze the dissolution mechanism of carbonate was conducted by using the rock core as experimental samples.The results show that:the mineral composition and reservoir space type of carbonate rock control selective dissolution.In the carbonate rock strata with high calcite content,the calcite is likely to dissolve first to form secondary dissolution pores;and the dissolution ability of gypsum(or anhydrite)in the carbonate rock is relatively weak in burial environment.Porous carbonate is mainly about the dissolving of the matrix pore,with porosity and permeability increasing correspondingly,but the growth is not obvious;in comparison,dominant channels for fluid are likely to occur in fractured carbonate,resulting in a relative reduction in the dissolution volume,but great increase of permeability.The corrosion ability of carbonate rock in acid fluid increases first and then decreases with the rise of temperature and pressure,indicating that there is an optimum range of temperature and pressure for the dissolution of carbonate rock.Based on the sedimentary and tectonic characteristics of Shaya Uplift,the model of the dissolution of the high-quality deep carbonate reservoirs is established in combination with the experimental results and the actual dissolution characteristics.It is found that syngeneic-penecontemporaneous atmospheric freshwater dissolution and weathering crust karstification are the basis for the development of reservoir space of high-quality deep carbonate reservoirs.The organic acid fluid plays an important role for the reservoir space by remoulding it in burial environment,in particular,the calcite which is not completely dolomitizated is easy to be dissolved preferentially.The contribution of hydrothermal fluid to reservoir space is weak even though there developed hydrothermal dissolution,because the hydrothermal fluid mainly moves along the fracture,and the dissolution ability is weakened at high temperature and pressure leading to the reservoir space filled with hydrothermal mineral.Finally,according to the results of dissolution simulation experiment and the problems existing in the current research on simulation experiment,suggestions or opinions for the problems of the karst of complex carbonate rock,the selection for solution fluid,the seepage direction of fluid,and the change of water-rock reaction area are put forward in this paper with an aim to promote the development of carbonate dissolution simulation experiment.