洞庭盆地生物气地球化学勘探及资源远景评价

湖南省洞庭盆地具备形成一定规模生物气藏的条件.针对生物气资源勘探,2013~2016年在盆地内开展了以地球化学为主的勘探工作,进行了比较系统的地表沉积物测量.勘探涉及面积2 060 km2,采集样品1 498个,采用现场顶空气游离烃、室内酸解烃和少量微量甲烷同位素指标测试.研究结果表明:①洞庭盆地甲烷地球化学异常由游离烃(顶空气)和吸留烃(酸解烃)组成.游离烃是现存地下生物气的动态反映,酸解烃与沉积相造成的碳酸盐沉积和古河道分布有关.②洞庭盆地为甲烷指标高背景区域,异常分布与第四纪断陷盆地分布基本吻合,尤以沅江凹陷显著.③酸解烃河坝镇异常与中部NE向串珠状和东南部次级局部异常组成的环状异常围绕着沅江凹陷,表明其可能是洞庭盆地潜在生物气烃源区.④沅江凹陷北部青树嘴—河坝镇一带是盆地内游离烃甲烷特高值异常区,是下覆生物成因天然气在浅表层直接渗漏的反映;酸解烃甲烷高值异常环绕着游离烃甲烷高值异常,构成了酸解烃环状异常和游离烃顶端异常最佳油气藏组合模式.青树嘴—河坝镇区块是洞庭盆地下覆生物气的“烟囱”,是本区生物气勘探最有可能突破的地区.

Geochemical exploration and resource potential evaluation of biogenic gas in Dongting Lake Basin

Dongting Lake Basin has the conditions for forming biogenic gas reservoirs of certain scales.For the exploration of biogenic gas resources,the authors carried out a comprehensive exploration work mainly based on geochemical methods during the period of 2013 ~ 2016,and conducted fairly systematic surface sediments survey.Geochemical exploration covered an area of 2060 km2,and collected 1498 samples.All the samples were tested by in situ headspace gas (free hydrocarbon),indoor acidolysis hydrocarbon and a small quantity of methane isotope indexes.The results are as follows:1.The geochemical anomaly of methane in the Dongting Lake basin is composed of free hydrocarbon (headspace gas) and adsorbed hydrocarbon (acid hydrolysis hydrocarbon).The free hydrocarbon is a dynamic reflection of the existing underground biogenic gas,and the hydrocarbon accumulation of the acidolysis hydrocarbon is related to the distribution of the paleo-channel.2.The Dongting Lake basin is the area of a high background and high anomaly of methane,and the distribution of the abnormal area is basically consistent with the distribution of the Quaternary fault basin,especially in Yuanjiang sag.3.Acidolysis hydrocarbon anomaly of the Heba Town,bead-like anomaly in NE direction and southeastern secondary local anomalies form a ring anomaly zone surrounding the Yuanjiang sag,which means that the sag may be the potential supply area of biogenic gas in Dongting Lake basin.4.The area of Qingshuzuie-Heba Town north of Yuanjiang sag is the abnormal high value area of the hydrocarbon methane in Dongting Lake Basin,and this is the direct reflection of natural gas seepage in shallow surface.The high value of acidolysis hydrocarbon methane surrounds the anomaly area of free hydrocarbon methane in Qingshuzui,which constitutes the best combinational hydrocarbon reservoir model.The area of Qingshui-Haba Town is the'chimney'of the biogenic gas underlying the Dongting Lake Basin and hence the most hopeful breakthrough area for biogas survey in the Dongting Lake Basin.