琼东南盆地中央峡谷深水大气田形成关键要素与勘探前景

琼东南盆地深水区新发现的大气田位于中央峡谷乐东-陵水段,具有气层厚度大、储层物性好、纯烃含量高、单层测试产量高等特点。研究认为煤系烃源岩供烃、重力流水道砂岩储集、底辟近源垂向运移是形成中央峡谷深水大气田的三大决定性因素。古近系崖城组海陆过渡相煤系烃源岩广泛分布,成熟度高,是主力烃源岩;高温高压凹陷深埋藏烃源岩仍具较好的生烃潜力。中央峡谷是南海北部深水区独特的重力流沉积体系,具有分段式发育、多物源汇聚、多期次充填特征,重力流水道砂形成了该深水区厚层优质储层;且水道砂岩被后期泥质水道切割、块体流改造,与峡谷壁联合形成了多个岩性圈闭。深部异常高压促使凹陷内底辟、裂隙比较发育,底辟作为垂向高效沟源通道,油气可直接运移至中央峡谷内的岩性圈闭聚集成藏。深水区烃源条件优越、储集条件良好、勘探目标成群成带分布,天然气总潜力超2×10¹²m³,具有巨大的勘探潜力;乐东-陵水凹陷以及宝岛-长昌凹陷的海底扇形成的构造-岩性圈闭群是继中央峡谷之后的深水区下步有利勘探领域。

Critical factors for the formation of large-scale deepwater gas field in Central Canyon System of Southeast Hainan Basin and its exploration potential

The newly-discovered large-scale deepwater gas field in Southeast Hainan Basin is located at Ledong-Lingshui segment of Central Canyon System (CCS), characterized by considerable thickness of gas layers, favorable physical properties of reservoirs, high pure hydrocarbon content and high test yield for monolayer. Studies have shown that there are three decisive factors for the formation of large-scale deepwater gas field in CCS, i.e., hydrocarbon supply of coal-bearing source rocks, sandstone accumulation in gravity flow channel and proximal vertical migration of diapir. Coal-bearing source rocks are widely distributed in the marine-continental transitional facies of Paleogene Yacheng Formation, which are chief source rocks with high maturity. The source rocks with large buried depths in high temperature and high pressure depressions still have favorable hydrocarbon generation potential. CCS is a unique gravity flow sedimentary system in deepwater region of the northern South China Sea, characterized by segmental development, multiple-source confluence and multi-stage fillings. The gravity flow channel sandstones lead to the formation of high-quality thick reservoirs in this deepwater region. Meanwhile, channel sandstones are cut by muddy channels and transformed by mass flows in the late period, thus forming multiple lithologic traps in combination with canyon walls. Due to abnormally high pressure in deep positions, diapirs and fractures are relatively developed in the depression. Diapirs are effective vertical channel-source channels, so that hydrocarbons can vertically migrate to the lithologic traps of CCS for accumulation. The deepwater region has excellent source rock conditions, favorable accumulation conditions, as well as clumped and zonal distribution of exploration targets, where the total natural gas is more than two trillions with a huge exploration potential. Moreover, the structural-lithologic trap group of submarine fans in Ledong-Lingshui depression and Baodao-Changchang depression is the next exploration target of deepwater region followed by CCS.