四川盆地东部下寒武统筇竹寺组烃源岩评价

Evaluation of source rocks of Lower Cambrian Qiongzhusi Formation in eastern Sichuan Basin

四川盆地下寒武统筇竹寺组含油气系统已获探明储量超过1.3×1013 m 3,但川东地区经历30年勘探仍未取得实质性突破。为厘清川东地区筇竹寺组烃源岩生烃条件,对新近钻井楼探1井、五探1井、马深1井、焦石1井和城口剖面等开展有机地球化学综合研究。结果表明:马深1井筇竹寺组厚度为358 m,底部黑色泥岩厚度约130 m;向北至盆地外缘杨坝剖面筇竹寺组厚度增加,总厚约400 m,但下部泥岩,总有机碳含量(质量分数)TOC>0.5%,厚度减小到50 m;川东地区筇竹寺组厚度110~150 m,下部黑色泥岩厚度约60 m。川北马深1井—福成地区筇竹寺组平均TOC含量为3.39%~3.44%;川东楼探1井—城口地区平均TOC含量相对较小,为0.52%~2.58%。盆地周缘筇竹寺组剖面等效镜质体反射率(R o)在2.22%~2.53%,盆地内钻井R o在2.57%~3.10%;受古埋深影响,热演化程度由盆地外向盆地内呈增加趋势。进一步结合黑色泥岩产烃率、原始有机碳恢复系数等相关参数,计算得到川东地区生气强度由盆地内向东北方向呈现增加趋势,鹰探1井—楼探1井区生气强度在(10~20)×10^(8) m^(3)/km^(2);生气强度最大地区为城口区域,其生气强度达60×10^(8) m^(3)/km^(2)。结合典型钻井埋深沉降热史模拟表明,筇竹寺组烃源岩于志留纪(430 Ma)开始大量生烃,到三叠纪(250 Ma)进入生气阶段。综合研究表明,川东地区巴中—南江与城口地区两个生烃中心为下寒武统筇竹寺组含油气系统最有利生烃富集带。

The oil and gas system in the Qiongzhusi Formation in the Sichuan Basin from the Lower Cambrian has more than 1.3 trillion m 3 of reserves,but no substantial breakthrough in this regard has been made in the region after 30 years of exploration.This study comprehensively investigates the organic geochemistry of Well Loutan 1,Well Wutan 1,Well Mashen 1,Well Jiaoshi 1,and the Chengkou profile to determine the conditions of hydrocarbon generation by source rocks in the Qiongzhusi Formation in eastern Sichuan.The results showed that the Qiongzhusi Formation in Well Mashen1 was 358 m thick,while the black mudstone at its bottom was about 130 m thick.To the north,the thickness of the Qiongzhusi Formation increased to about 400 m while that of the lower mudstone(TOC>0.5%)decreased to 50 m.The formation was 110~150 m thick in the eastern Sichuan Basin while the lower black mudstone was about 60 m thick.Its average total organic carbon(TOC)content in the Well Mashen 1-Fucheng area of northern Sichuan was 3.39%~3.44%,while its average TOC content in the Well Loutan 1-Chengkou area of eastern Sichuan was relatively low at 0.52%~2.58%.The value of R o of the Qiongzhusi Formation around the basin ranged from 2.22%to 2.53%,while the R o of drilling in the basin was 2.57%~3.10%.Due to the depth of burial of the rock,the degree of thermal evolution increased from outside to inside the basin.We used the rate of hydrocarbon generation of black shale and the original coefficient of recovery of organic carbon to determine that the intensity of gas generation in the eastern Sichuan Basin exhibited a trend of increase from the basin to the northeast,and the intensity of gas generation in the Well Yingtan 1-Well Loutan 1 area was(10~20)×10^(8)m^(3)/km^(2).The Chengkou area had the highest intensity of gas generation along the periphery of eastern Sichuan,with a value of 60×10^(8)m^(3)/km^(2).We simulated the thermal history of the typical depth of burial of wells to show that the source rocks for hydrocarbon in the Qiongzhusi Formation had begun generating a large amount of hydrocarbons in the Silurian(430 Ma),and entered the stage of gas generation in the Triassic(250 Ma).Our results showed that the two centers of hydrocarbon generation in the Bazhong-Nanjiang area and Chengkou area in eastern Sichuan were the most favorable zones of enrichment in the petroleum system of the Lower Cambrian Qiongzhusi Formation.