CAJ | 학술논문

四川盆地上三叠统须家河组煤系是套陆相含煤地层.须家河组一、三、五段以暗色泥岩和煤为主,是气源岩.泥岩干酪根以Ⅱ型和Ⅲ型为主,有机碳含量平均为1.96%,在川中处于成熟阶段,在川西处于高成熟阶段.由于源岩以腐殖型为主,故以形成气为主,气中仅有少量轻质油或凝析油.须家河组二、四、六段以砂岩为主,故有3套生储盖组合,形成许多自生自储煤成气田.在四川盆地须家河组发现的天然气储量仅次于下三叠统飞仙关组,并有该盆地第二大气田(广安气田).须家河组煤成气碳同位素特征:一是绝大部分具有正碳同位素系列,即δ~(13)C_1<δ~(13)C_2<δ~(13)C_3<δ~(13)C_4;二是δ~(13)C_2值是全盆地9个产气层系中最重的,为-20.7‰～-28.3‰;三是川中地区有一批轻的δ~(13)C_1值,最轻为-43.0‰.在川东和川南须家河组变薄的地区还发现少量油型气藏,这些气藏碳同位素特征是δ~(13)C_2值轻,一般轻于-30‰,最轻为-36.3‰,易与煤成气区分.
사천분지상삼첩통수가하조매계시투륙상함매지층.수가하조일、삼、오단이암색니암화매위주,시기원암.니암간락근이Ⅱ형화Ⅲ형위주,유궤탄함량평균위1.96%,재천중처우성숙계단,재천서처우고성숙계단.유우원암이부식형위주,고이형성기위주,기중부유소량경질유혹응석유.수가하조이、사、륙단이사암위주,고유3투생저개조합,형성허다자생자저매성기전.재사천분지수가하조발현적천연기저량부차우하삼첩통비선관조,병유해분지제이대기전(엄안기전).수가하조매성기탄동위소특정:일시절대부분구유정탄동위소계렬,즉δ~(13)C_1<δ~(13)C_2<δ~(13)C_3<δ~(13)C_4;이시δ~(13)C_2치시전분지9개산기층계중최중적,위-20.7‰～-28.3‰;삼시천중지구유일비경적δ~(13)C_1치,최경위-43.0‰.재천동화천남수가하조변박적지구환발현소량유형기장,저사기장탄동위소특정시δ~(13)C_2치경,일반경우-30‰,최경위-36.3‰,역여매성기구분.
The coal measures in the Upper Triassic Xujiahe Formation in Sichuan Basin are coal-bearing continental strata.Its first,third,and fifth members are dominated by dark mudstone and coal and are gas source rocks.The kerogen of the mudstone are mainly of type Ⅱand Ⅲ with an average TOC of 1.96%.They are mature in the central Sichuan Basin and are highly mature in the western Sichuan Basin.Since the organic matter of source rocks are dominated by huminite,the source rocks are gas prone and the gas generated only contains minor light oil or condensate.The second,fourth and sixth members consist mainly of sandstone.There are three sets of source-reservoir-caprock combinations,resulting in many indigenous coal-derived gas fields.The gas reserves discovered in the Xujiahe Formation is only next to that in the Lower Triassic Feixianguan Formation.The Guangan gas field,the second largest gas field in the basin,produce gas from the Xujiahe Formation.The carbon isotope of the coal-derived gas have the following features:1) most of them are positive carbon isotopes (i.e.,δ~(13)C_1<δ~(13)C_2<δ~(13)C_3<δ~(13)C_4),2) its δ~(13)C_2 value is between -20.7‰--28.3‰,the heaviest among that of the 9 gas pays in the whole basin,3) there are lighter δ13C1 in central Sichuan Basin with the lowest value of -43.0‰.A few gas reservoirs sourced from oil-prone kerogens had been discovered in eastern and southern Sichuan Basin,where the Xujiahe Formation thins out.These reservoirs have light carbon isotopes (typically lighter than-30‰,and in extreme cases as light as-36.3‰) and can be easily distinguished from coal-derived gases.