桂中坳陷海相地层油气成藏与热作用改造

桂中坳陷是在加里东运动基础上形成的晚古生代海相大型沉积坳陷,已演化成为由上古生界和三叠系充填的残留盆地,其中泥盆系深埋地腹,是主要的勘探目的层系。桂中1井揭示泥盆系—石炭系具有两套生储盖组合,钻遇了气测异常、油迹砂岩、固体沥青等三类油气显示,其中泥盆系沥青显示井段累计709m。桂中坳陷古油藏(沥青)主要来源于中—下泥盆统塘丁组和罗富组泥岩。桂中1井和南丹大厂储层固体沥青为运移/成藏的油气发生热裂解而形成的焦沥青。桂中1井经历了两期油气充注,第一期为印支期前的主力生烃期,第二期(晚期)为燕山期—喜马拉雅期,以紧邻油气显示层的可溶烃类为代表。经历了三期油气改造:第一期为印支期前的油气藏经历了地层沉降增温的热裂解作用而演化为固体沥青和甲烷天然气,第二期为燕山晚期岩浆活动导致的更高温度的热蚀变作用,第三期为喜马拉雅期构造抬升的改造或破坏作用对上泥盆统油气藏的影响。晚期成藏的天然气应是下一步的勘探方向。桂中坳陷西部地层保存相对完整(目的层深埋),断裂和岩浆活动相对不发育,可能更有利于天然气的聚集和保存。

Hydrocarbon Accumulation and Thermal Reconstruction of Neopaleozoic Marine Strata in Guizhong Depression

The Guizhong Depression in middle Guangxi province is a large Neopaleozoic marine sedimentary one that was developed on the base of Caledonian Movement and it has been evolved into a residual basin filled with Neopaleozoic and Triassic strata.The Devonian strata buried deeply are the main exploration targets.The drilled result demonstrates the existence of two suits of Devonian-Carboniferous source-reservoir-cap associations in Well Guizhong-1,in which three kinds of hydrocarbon showings were discovered,i.e.gas-testing abnormality,oil-traced sandstone and solid bitumen.The solid bitumen-showing intervals are thick up to 709m in Devonian strata.The hydrocarbon(bitumen)of paleo-reservoir mainly derives from middle-lower Devonian Tangding and Luofu mudstone in Guizhong Depression.The solid bitumen in Well Guizhong-1 and the paleo-reservoir at Dachang is pyrobitumen,one of the thermal cracking products of hydrocarbon.Two phases of hydrocarbon charging underwent in the trap of Guizhong-1.The first was during the dominant one of hydrocarbon generation before Indo-Chinese Epoch,and the second(the late phase)was during Yanshan-Himalayan Epoch,which is represented by the dissoluble hydrocarbon nearby with the hydrocarbon showing interval.Three times of hydrocarbon reservoir reconstructions went through in this depression.During the first,the hydrocarbon that accumulated before Indo-Chinese Epoch underwent thermal cracking due to subsidence and was changed into solid bitumen and methane gas.During the second,the thermal alteration of hydrocarbon happened under higher temperature caused by Later Yanshan magmatic activity.During the third,the gas formed in the reservoirs due to Upper Devonian reservoir reconstruction or destruction caused by Himalayan Movement.The gas accumulated during late phase is the future direction of exploration.The western part of Guizhong Depression is an area favourable for gas accumulation and preservation owing to relatively integrated strata(so the exploration targets are buried deeply)and undeveloping faults and magma activities.