Analysis on geological conditions of tight gas accumulation in carbonate rock:a case study of Mao 1 Member in Da'an block,Sichuan Basin
Natural gas has been discovered successively in the limestone of Mao 1 Member in Da'an block of Sichuan Basin,including Well Da'an 2,Well Da'an 1,and Well Daba 1.However,geological conditions of natural gas accumulation are unclear because of the low degree of exploration.Taking Mao 1 Member in Da'an block as an example,combining field geological survey,single well analysis,organic carbon analysis,thin section identification,X-ray diffraction,field emission scanning electron microscopy and other test results,the source rock lithology,geochemistry charateristics,reservoir sedimentary facies,mineral component,micro pore characteristics and gas bearing system analysis are studied,geological conditons of tight gas accumulation of Mao 1 Member in Da'an block are analyzed,and the reservoir forming model is established.The results indicate that the Mao 1 Member eyeball and eyelid limestone deposits are controlled by the middle gentle slope sendimentary environment,which forms the material basis for self-generation and self-accumulation.Eyelid limestone is a set of effective source rock,which ensures sufficient supply of hydrocarbon source.The eyeball and eyelid limestone are tight reservoirs with low porosity,low permeability,displaying interlayered stacking.Nano and micro pores are developed in the reservoir with interconnected micro cracks,which are conducive to large-scale reservoir formation The reservoir forming model is tight gas source-reservoir integration,in-source accumulation,and nearby adsorption.Natural gas is dynamically generated and gathered,which is not easy to escape.The area was formed in the Middle and Late Yanshan period and preserved during the Himalayan period.The eyelid limestone of the middle gentle slope facices has characteristics of high brittleness,medium high TOC and medium gas content,making it a favorable direction for tight gas exploration.
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