延长气田上古生界烃源岩评价及生排烃特征

延长气田位于鄂尔多斯盆地一级构造单元陕北斜坡中南部,是一个新探明的天然气富集区,对烃源岩的研究评价工作相对较薄弱,制约了该区的油气勘探进展。为此,通过对大量新探井岩心样品的有机质丰度、有机质类型、有机质成熟度等资料的综合分析,探讨了该气田上古生界烃源岩的展布特征、地球化学特征和生排烃特征。结果表明:①煤岩为该区最好的烃源岩,其有机碳含量介于60.18%~86.89%,累积厚度介于6~18 m,是该区上古生界天然气最主要的烃源岩,而泥岩则是天然气的第二位供应者,其中山西组泥岩属中等—很好的烃源岩,太原组和本溪组泥岩为好很好的烃源岩;②烃源岩干酪根的类型主要为腐殖型,其R。多大于2.0%,已进入过成熟—干气阶段;③上古生界烃源岩在中晚侏罗世—早白垩世末处于大量生排烃期,现今生烃强度介于(25~65)×10~8m~3/km~2,现今排烃强度介于(15~60)×10~8m~3/km~2,煤的生排烃强度大干泥岩。

Quality evaluation and hydrocarbon generation-expulsion characteristics of source rocks in Upper Paleozoic in the Yanchang Gasfield,Ordos Basin

The Yanchang Gasfield is located in the south central part of North Shaanxi Slope (the first-order structural unit), Ordos Basin. Although natural gas is abundant in this newly discovered area, there is little research related to the Upper Paleozoic source rocks. Consequently, progresses in exploration of the area are limited. Through comprehensive analyses of the organic matters, organic matter types, organic matter maturity and other properties, the distribution patterns, geochemical characteristics and hydrocarbon generation-expulsion properties of the Upper Palaeozoic source rocks were investigated. From the research, the following conclusions were made. ①With organic carbon content of 60.18% - 86.89% and total thicknesses of 6 - 18 m, coals are the major gas source rocks within the Upper Palaeozoic gas reservoirs, while mudstone is the second important gas supplier. Typically, mudstone in Shanxi Formation is classified as medium-high quality source rocks, while mudstone in Taiyuan and Benxi Formations serve as good-very good source rocks. ②Kerogen in source rocks is dominated by humus type with R_o over 2.0%, which indicates a stage from over maturity to dry gas. ③Source rocks in Upper Palaeozoic entered a massive hydrocarbon generation-expulsion stage in Middle-Late Jurassic-end of Early Cretaceous. Currently, hydrocarbon generation intensities vary at (25 - 65)×10⁸ m³/km² and expulsion intensities vary at (15 - 60)×10⁸ m³/km². In conclusion, coals have better hydrocarbon generation-expulsion capacities than mudstones.