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川西北区块上三叠统须五段广泛分布，其下部有上、下两个砂岩段，分别厚24．4－28．4m及145．8－92．8m。综合地质及测井解释须五储层具有深盆气特征。该区的区域构造可分为中央隆起、东部凹陷及西部凹陷三带，东部凹陷西坡的思依1井获工业气流。产气段底界海拔低于中央隆起带27口井段五段底。选井进行测井解释，中央隆起带各井储层孔隙度、地层水电阻率及含水饱和度高，为水层；东西凹陷带井的储层孔隙度、地层水电阻率及水饱和度低，为气层或气水层。中央隆起带地层压力系数为1．746－1．8569，西凹陷大于2，东凹陷近于1，形成了高低两个异常区。该区须三、须五段及白田坝组煤层发育，煤成气为气藏主要烃源。认为东缘的思依1井须五气藏属于深盆气藏，广大的东部凹陷为含气的深盆区。西部凹陷也具有深盆区特征.

Characteristics of Deep-basin Gas in T3x5 Reservoir in Northwest Sichuan Tract

??The fifth member of Xujiahe Formation of Upper Triassic(T₃x⁵)is widely distributed over all the Northwest Sichuan tract and in its lower part,there are up-and-down two sandstone intervals with thicknesses of 24.4??28.4 m and 92.8??145.8m respectively.According to the comprehensively geological and logging interpretation,the gas found in T₃x⁵ reservoir is of the characteristics of deep-basin gas.Tectonically this tract may be divided into three belts,i.e.central uplift belt,east seg belt and west seg belt.Commercial natural gas was found in the well Siyi-1 at the west slope of the east seg and the elevation of the bottom surface of gas-producing interval is below that of the bottom of T₃x⁵ in each of the 27 wells at the central uplift belt.In light of log data,the reservoirs in various wells at the central uplift belt are water-bearing strata,being of high porosity,high formation water resistivity and high water saturation;and those at the east and west seg belts are gas-bearing or gas-and water-bearing strata,being of low porosity,low formation water resistivity and low water saturation.The formation pressure coefficients of the central uplift belt range from 1.746 to 1.857,those of the west seg belt are more than 2 and those of the east seg belt near 1,two(high/low)pressure abnormal areas being formed.Because the coal beds are well developed in T₃x³,T₃x⁵ and Baitianba Formation in this tract,the coal-formed gas is the major hydrocarbon source of the gas reservoirs.It is considered that the T₃x³ gas reservoir found by the well Siyi-1 belongs in a deep-basin gas reservoir;the extensive east seg is a gas-bearing deep-basin area;and the west seg is also possessed of the characteristics of the deep-basin area.