藏北羌塘盆地双湖地区油气成藏条件

Conditions for the formation of petroleum accumulations in the Shuanghu area,Qiangtang basin,northern Tibet

藏北双湖地区油气显示有中侏罗统布曲组的隆鄂尼碳酸盐岩古油藏、充填于夏里组石膏孔洞里的沥青和夏里组砂岩夹层中的油页岩。布曲组及索瓦组中的生油岩有机碳含量普遍较高,生油能力大。油页岩的氯仿沥青“A”高达25255×10-6,有机碳含量28.8%,但总烃含量在40%～50%左右,OEP值分别为0.7和0.8,显示有机质演化处于低成熟阶段。有机质中既有Ⅰ型酐酪根,也有Ⅲ型酐酪根。有机质形成环境既有浅海陆棚相开阔型水体环境,也有半封闭型泻湖相等沉积环境。该地区可划分出4套生、储、盖组合,以第一套组合(由曲色组和布曲组组成)为最好。

Oil and gas showings in the Shuanghu area,northern Tibet,show that there are the ancient Long'eni carbonate accumulation in the Middle Jurassic Bi Qu Formation,bitumen filled in gypsum pore spaces of the Gyari Formation and oil shale developed in sandstone intercalations of the Gyari Formation.The source rocks of the Bi Qu and Suowa Formations have high organic carbon contents and a high ge-netic potential.The content of chloroform bitumen“A”in oil shale is as high as25255×10 -6 and its content of organic carbon is28.8%,but the total hydrocarbon yield is40%～50%.The OEP values are0.7and0.8respectively,suggesting that the organic matter evolution remains at a low-mature stage.There are both type-Ⅰkerogen and type-Ⅲkerogen in organic material.The environments for the formation of organic material include neritic shelf open water environment and semi-enclosed lagoonal sedimentary envi-ronment.Four associations of source rocks,reservoir rocks and cap rocks may be distinguished in the Shuanghu area,northern Tibet,of which the first association(consisting of the Bi Qu and Quse Forma-tion)is the best.