鄂尔多斯盆地海相烃源岩系发育特征与勘探新领域

鄂尔多斯盆地油气勘探已进入储量发现高峰期,围绕延长组烃源岩的石油资源和来自上古生界煤系地层的天然气资源基本探明。随着盆地油气勘探的深入,急需寻找新的油气勘探接替领域。研究认为,古老海相烃源岩发育2套:即中上奥陶统海相碳酸盐岩烃源岩系和中上元古界海相碎屑岩烃源岩系。中上奥陶统海相烃源岩在不同海域有不同的特征。中奥陶统海相烃源岩发育于盆地东部华北海台内盐坳周边,以泥灰岩为主,单层厚度薄,但层系多,有机质丰度差—中等,高成熟度,有规模生烃潜力,对奥陶系盐下自源气藏有重要贡献。上奥陶统海相烃源岩主要发育于盆地西部和南部"L"型秦祁海槽斜坡相带,以泥页岩、泥灰岩为主,有机质丰度中等,累计厚度大,在天环凹陷上达到高—过成熟生气阶段,分布范围有限,在盆地西缘逆冲带上盘处于成熟生油阶段,生烃潜力较大。而中上元古界海相泥岩烃源岩系在邻区为有效烃源岩,以泥板岩为主,厚度大,有机质丰度中等,高过成熟度。在盆地本部甘陕裂陷槽新钻井中也有部分揭示。围绕2套海相烃源岩系分布的奥陶系海相碳酸盐岩和中上元古界深层海相层系是未来天然气勘探的2个新领域,其中5个有利勘探区带天然气勘探潜力大,有望形成天然气勘探接替区带。

Development characteristics and new exploration areas of marine source rocks in Ordos Basin

Oil and gas explorations in the Ordos Basin have already entered the peak period of reserves discovery. The reserves of oil resources in the source rocks of Yanchang Formation and natural gas resources in Upper Palaozoic coal measure strata have been basically ascertained. As the explorations of oil and gas in the basin progresses, it is a fairly urgent task to find new replacement exploration areas. According to the previous research results, the ancient marine source rocks are well developed in this region, including the Middle-Upper Ordovician marine carbonate source rock series and the Middle-Upper Proterozoic marine clastic source rock series. The Middle-Upper Ordovician marine source rocks exhibit different characteristics in different waters. The Middle-Upper Proterozoic marine source rocks are developed around the salt depression of Huabei Submarine Plateau in the eastern Basin, dominated by argillaceous limestones, with thin single layer and multiple series of strata. These source rocks have low-moderate abundance of organic matter and high maturity; moreover, they have large-scale hydrocarbon generation potential and make a great contribution to the Ordovician pre-salt self-source oil reserves. The Upper Ordovician marine source rocks are mainly developed in the L-shaped slope facies of the Qin-qi Trough in the western and southern Basin, dominated by shales and argillaceous limestones. These source rocks are characterized by moderate organic matter abundance and great cumulative thickness. They reached the high-postmature gas generation stage in Tianhuan Depression, with a limited distribution range. Additionally, these source rocks entered in a mature oil generation stage on the hanging side of the thrust belt along the western margin of the Basin, exhibiting great hydrocarbon generation potential. The Middle-Upper Proterozoic marine argillaceous source rocks are effective source rocks in the adjacent region, dominated by argillites and featured by moderate organic matter abundance and high postmaturity, which is also partly revealed in some new wells at the Gan-Shan Taphrogenic Trough in the Basin. The Ordovician marine carbonatites and the Middle-Upper Proterozoic deep marine strata in which the above-described two marine source rocks are extensively distributed are two new areas of natural gas exploration in the future, in which five favorable zones exhibit great exploration potentials and are expected to become the replacement zones of natural gas exploration.