| Due to the arid and hot climate,the shallow water bodies lead to massive deposition of gypsum salt rocks.Gypsum salt rock is widely distributed in the Cambrian sedimentary period of Sichuan basin,especially during the long Wang Miao formation and Gao Tai formation deposition period.Many experts have researched on gypsum petrological characteristics,gypsum distribution and oil and gas accumulation of Cambrian in Sichuan basin,but there still are some controversies about sedimentary environment,causes,reservoirs and caps effection,which have important significance on oil and gas exploration in Sichuan basin.Therefore,it can provide reliable geological evidence for finding the favorable oil and gas reservoir under salt in Cambrian of Sichuan basin,through analysing the distribution regulation of gypsum salt rock,the deposition environment,effectiveness of the sealing capacity and the relationship between gypsum salt rock and reservoir development.Gypsum salt rock mainly contains three components: dolomite,gypsum and terrigenous detrital mineral.Based on the size of three elements contents,gypsum salt rocks can be divided into salt gypsum rock,gypsum dolomite and gypsum mud(sand)rocks and halite rocks,and then these four categories can be further divided into eight types: pure gypsum rocks,gypsum karst breccia,dolomitic-bearing gypsum rocks,marble gypsum rocks,gypsum dolomite,gypsum-bearing dolomite,gypsum-bearing mud(sand)rocks,gypsum.Gypsum rocks of cambrian Sichuan basin are mainly developed in the lower Cambrian Long Wangmiao formation and middle Cambrian Gao Tai formation,but gypsum rocks are less developed in the upper Cambrian sedimentary period.The cambrian gypsum rock thicknesses in the study area are between 0 and 419.9m,with wide variation ranges and two sedimentary centers.The thickness contours of the western part of the study area generally distribute north-east to west,and gradually move eastward to the east and the north.The gypsum rock thicknesses in middle cambrian is between 0 and 675 m,which has a very large range of change and also have two sedimentary centers.The gypsum salt rock thicknesses are distributed with the direction of NE to NW in southwest of Sichuan-northern Guizhou regioneastern Sichuan and western Hubei.And gypsum salt rock thicknesses are distributed with the direction of NW to SE in the southwest of Sichuan and northern of Guizhou.According to the lithology characteristics and distribution rules of gypsum salt rock in the study area,its sedimentary environment can be divided into supratidal Sabkha,intertidal gypsum salt lake and subtidal limited lagoon.The Supratidal Sabkha mainly developed flats of the supratidal zone with low energy,which would be drowned in the tide and storm tide.The rock types include light grey-grey mud dolomite,grey quartz sandstone,purple red mudstone,yellow gypsum breccia and shallow purple gypsum rock or salt rocks.The continuous deposition thicknesses of gypsum salt rocks are usually less than 15 m,with exposed structure and wavy stromatolite.The intertidal gypsum salt lake mainly developed in intertidal zone and low shallow with low energy.The rock types are mainly gray and dark gray grain dolomite,white gypsum rocks and gypsum dolomite,occasionally depositing thin layers of light grey limestone and grain dolomite.The continuous deposition thicknesses of gypsum salt rocks are from 15 to 50 m,with wavy stromatolite and less exposed structure.The subtidal limited lagoon developed in deep water and sunken of limited subtidal.The rock types are mainly gray or bluish blue argillaceous dolomite,dark gray dolomite,dark green shale,shallow red gypsum,gypsum dolomite and less deep gray grain limestone.The continuous deposition thicknesses of gypsum salt rocks are more than 50 m,without bedding structure.Combining with the sedimentary characteristics and analysis datas of gypsum salt rock,it is found that gypsum rocks have two sides influence on reservoirs.The constructive effects are dissolution and dolomitization and the destructive effects are gypsum mineral filling and gypsum cementation.The dissolution of gypsum rocks has great influence on reservoirs development.The gypsum rocks in the southwest of Sichuan has been dissolved and their reservoir property is better.The reservoirs are mainly distributed between Nanchong-Ziyang and Hechuan-Weiyuan.But the gypsum rocks in southeast of Sichuan have no tdissolved and its reservoirs are poor.The reservoirs are mainly distributed between Nan Chuan and Xi Shui.Gypsum salt rocks are ideal reservoir cap rocks,and its sealing performance is mainly affected by brittleness and tectonics.Gypsum rocks of middle cambrian series entered into the plastic stage at permian sedimentary period.Earlier than the lower hydrocarbon source rocks hydrocarbon expulsion phase,so it is better for hydrocarbon accumulation.Southeast areas of Sichuan are effected by Qi Yaoshan fault,enclosing cap performance is favorable.Mainly gypsum salt distributions from Xi Shui to Nan Chuan.in Southwest of Sichuan are influenced by faults;some places located in Chuan Zhong uplift subjected develop corrosion.And the cap capacities in Wei Yuan–Fan Dian areas are favorable,which are the main caps of Wei Yuan gas fields.Synthesizing the generating,reservoiring and capping conditions of gypsum salt rocks,comprehensive condition of reservoir-cap and distribution range of source rocks,under salt exploration area can be divided into three categories.They are the most favorable area,the more favorable area and the general zone.The most favorable exploration area is between Moxi-Gaoshiti-Weiyuan.It located in high ground of carbonate platform,thethickness of paticles dolomite is bigger.And it mainly effected by constructive diagenesis,the main cap rock is the argillaceous limestone of the upper Cambrian.The favorable areas are from Fan Dian to Ma Bian area and their sedimentary environments are supratidal hunji environment.Although environments with strong dissolution,but its porosity affecting by the terrigenous clasts and fillings is worse than porosity in Moxi-Gao Shiti-Weiyuan areas.What's more,the Fan Dian-Ma Bian arean can be favorable area due to the effective caps of the gypsum salt rocks.The sedimentary water in Xishui-Nanchuan line is deep and the primary porosity of mud dolomite is poor.Although the subtidal shoals affecting by the low uplift in western side of the Qi Yaoshan are very developed,but its pores are filled with dolomite and gypsum,and the diagenesis constructive is weak.What's more,gypsum salt caps are influenced by the fault zone in Qi Yaoshan,which let oil and gas lost.Therefore,Xishui-Nanchuan areas just can be general zone. |
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