鄂尔多斯盆地奥陶系马家沟组规模性有效烃源岩的发现及其地质意义

Discovery of effective scale source rocks of the Ordovician Majiagou Fm in the Ordos Basin and its geological significance

尽管对于鄂尔多斯盆地奥陶系马家沟组天然气的来源问题一直都存在着不同的认识,但所有观点均不认为马家沟组自身发育TOC〉0.5%的规模性烃源岩。通过分析该盆地中东部奥陶系近年来大量新钻井的岩心和岩屑样品,揭示了马家沟组不同岩石类型烃源岩的有机质丰度特征,首次发现并证实马家沟组存在有机质丰度高的规模性有效烃源岩。研究结果表明：①马家沟组有效烃源岩的岩性主要为暗色薄层—厚层状含云泥岩、云质泥岩和泥云岩,其富集分布明显受沉积相的控制,马家沟组沉积期间海退期较海侵期更有利于规模性有效烃源岩的发育,层位上主要集中分布在马五上段,其次为马五中—下段、马三段和马一段;②平面上有效烃源岩围绕米脂盐洼呈双环带状分布,次级洼陷有效烃源岩呈中厚—厚层状,累计厚度大、有机质丰度高;③有效烃源岩在米脂凹陷中心和次级隆起则呈薄层状,累计厚度小、有机质丰度低;④有效烃源岩的有机碳含量变化范围为0.30%~8.45%,其生烃母质为浮游藻类和疑源类,有机质类型为腐泥型或偏腐泥混合型。结论认为：该盆地奥陶系有效烃源岩普遍处于过成熟阶段,以产干气为主,生气量大,是马家沟组天然气的主要贡献者。

There have been different views on the origins of gas reservoirs of Ordovician Majiagou Fm in the Ordos Basin, but none of them supports the opinion that there are scale source rocks （TOC〉0.5%） in the Majiagou Fm. In this paper, a series of analysis was made on cores and cutting samples taken from recent newly-drilled wells in the Ordovician reservoirs in the central-eastern areas of the Ordos Basin. Accordingly, the organic abundance features of different types of source rocks in the Majiagou Fm were presented, and for the first time, it was discovered and verified that there are effective scale source rocks with high organic abundance. Based on these analysis and studies, the following results were achieved. First, the effective source rocks of Majiagou Fm are composed of thin-thick layered dark dolomite-bearing mudstones, dolomitic mudstones and argillaceous dolomites, and their enrichment and distribution are obviously con- trolled by sedimentary facies. During the sedimentation of Majiagou Fm, the effective scale source rocks developed better at the regres- sive stage than at the transgressive stage. The effective source rocks are primarily distributed in the upper part of Ma 5 Member （espe- cially concentrated in the third and fourth sub-members） and secondly in Ma 3 Member, Ma 1 Member and the middle-lower part of Ma 5 Member. Second, the effective source rocks are areally distributed in a dual-girdle form around the Mizhi Salt Depression, and those in the secondary depressions are medium-thick layered, with a large total thickness and high organic abundance. Third, the effective source rocks in the center of Mizhi Depression and the secondary uplifts are thin-layered, with a small total thickness and low organic abun- dance. And fourth, the organic abundance of the effective source rocks varies from 0.30% to 8.45%. Phytoplankton and Acritarchs are the main parent materials of hydrocarbon, and organic matters are of sapropel or sapropel prone type. To sum up, the Ordovieian effective source rocks in the Ordos Basin are generally at an over-mature stage and they contribute greatly to the natural gas in the Majiagou Fm. In these effective source rocks, dry gas is mainly generated at a high rate.