鄂尔多斯盆地马家沟组盐下碳酸盐岩烃源岩分布特征

针对鄂尔多斯盆地中东部盐下奥陶系内幕天然气自生自储特征及其相应烃源岩发育不清的实际，利用地球化学与测井技术相结合的方法，分析了研究区奥陶系盐下烃源岩的分布特征。奥陶系马家沟组马五₆亚段膏岩最为发育，分布最为广泛，该膏岩层之下发现的自生自储天然气出气点表明了海相碳酸盐岩烃源岩具有一定的生排气能力。研究表明:研究区马家沟组盐下碳酸盐岩烃源岩有机质丰度总体偏低，测试的有机碳含量平均值约为0.2%，大部分低于0.5%；岩石热解T_ma 平均值达484℃,处于高成熟—过成熟阶段。平面分布上，盐下碳酸盐岩烃源岩有机碳含量在西部相对较高，向东逐渐降低；有机碳含量在北部相对较高，向盆地中部降低，最南部略有升高。研究区存在3个相对较好的烃源岩有利分布区。

Distribution characteristics of subsalt carbonate source rocksin Majiagou Formation,Ordos Basin

Aimed at the fact of the self-preserved and self-generated characteristics of the Ordovician inside-scene subsalt gas reservoir and ambiguous recognition of the development of its corresponding source rocks,the distribution features of the Ordovician subsalt source rocks in the study area have been analyzed by jointly employing organic geochemistry and well logging technology.The gypsum layers are most widely developed in the 6 sub-members of the member 5 of the Ordovician Majiagou Formation with the widest distribution.The gas production points of the self-stored and self-generated natural gas discovered below the gypsum layers indicate that the marine carbonate source rocks are entitle certain ability for gas generation and discharging.It is indicated in the study that generally the subsalt carbonate source rocks in the Majiagou Formation have low organic matter abundance with a measured mean TOC content of about 0.2%,most of them being lower than 0.5%.The average rock pyrolysis peak temperature reaches 484℃,seating in the stage of highly-mature to overmature.On plane,the subsalt carbonate source rocks show high TOC content in the west,decreasing gradually in the east.It is higher in north Ordos Basin and decreased to the center;however it finally increased in south Ordos Basin.There are three favorable areas for the distribution of relative source rocks in the study area.