准噶尔盆地东南部石炭系规模烃源岩的发现及天然气勘探潜力

Discovery of large-scale Carboniferous source rocks and natural gas exploration potential in the southeast of Junggar Basin

准噶尔盆地天然气资源丰富,但探明程度低。盆地东南部阜26井石炭系自生自储气藏的发现揭示了一个崭新的天然气勘探领域。准噶尔盆地东南部上石炭统巴塔玛依内山组发育一套煤系烃源岩,其岩性主要为碳质泥岩和煤系泥岩,含少量煤。碳质泥岩和煤的氢指数平均为182 mg/g,部分样品的有机显微组分中壳质组和腐泥组含量之和达20%以上,有机质类型为Ⅱ_(2)—Ⅲ型,以生气为主,兼具一定生油能力,总体属于中等—好的烃源岩。巴塔玛依内山组碳质泥岩和煤的Pr/Ph比值平均达4.14,C_(19)/C_(21)三环萜烷比值、C_(20)/C_(21)三环萜烷比值、C_(30)重排藿烷/(C_(30)重排藿烷+C_(29)Ts)比值以及C_(29)规则甾烷相对含量高,伽马蜡烷指数低,表明其形成于淡水氧化沉积环境,以陆源高等植物输入为主。煤系泥岩发育于高盐度的沉积水体中,水生生物输入更多,形成于氧化—还原的过渡环境。煤系泥岩的有机质类型和丰度较碳质泥岩略差,以Ⅲ型干酪根为主,这主要是由于其所处的低能海岸或海湾浅水区水体具有强烈的季节性变化所致。白垩纪末期,巴塔玛依内山组烃源岩在准噶尔盆地东南部凹陷区和斜坡区大面积成熟,现今已普遍进入高成熟—过成熟阶段,而凸起区现今仅有部分进入主生油窗。火山活动对烃源岩的形成和演化具有明显的促进作用:部分烃源岩中含有丰富的凝灰质组分且表现出较好的生烃能力;局部地区因岩浆烘烤而导致烃源岩的镜质体反射率异常高,并造成其生烃窗向浅部迁移。在准噶尔盆地东南部,石炭系巴塔玛依内山组烃源岩厚度大于100 m的范围达3835.4km2,在阜康凹陷东斜坡和吉木萨尔凹陷发育2个生气强度大于100×10^(8)m^(3)/km^(2)的生气中心。准噶尔盆地东南部整体上具备形成大—中型气田的资源潜力,有望成为继克拉美丽气田之后准噶尔盆地天然气勘探的重要接替领域。

The Junggar Basin is rich in natural gas resources but low in discovery level.The discovery of the Carboniferous self-generation and self-accumulation gas reservoir in Well Fu26in the southeastern part of the basin has revealed a brand-new exploration field of natural gas.A set of coal-measure source rocks are developed in the Upper Carboniferous Batamayineishan Formation in the southeast of Junggar Basin,whose lithologies are mainly carbonaceous mudstone and coal-measure mudstone,with a small amount of coal.In the carbonaceous mudstones and coal,the average hydrogen index is 182 mg/g,and the total content of exinite and sapropelinite in the organic maceral of some samples is more than20%.The organic matter of type Ⅱ_(2)-Ⅲ is dominant,mainly producing gas,with certain oil-generating ability,indicating medium to good source rocks as a whole.For the carbonaceous mudstone and coal in Batamayineishan Formation,the average Pr/Ph ratio is 4.14,the C_(19)/C_(21) and C_(20)/C_(21) tricyclic terpane ratios,the C30 diahopane/(C_(30) diahopane+C_(29) trisnorhopane)ratio and the relative content of C29regular steranes are high,but the gammacerane index is low,indicating that it was formed in a freshwater oxidative sedimentary environment with dominant input of terrestrial higher plants.Coalmeasure mudstones were developed in sedimentary water with high salinity and high input of aquatic organisms,and were formed in the transitional environment from oxidation to reduction.Coal-measure mudstone is slightly worse than carbonaceous mudstone in terms of the organic matter type and abundance,dominated by type Ⅲ kerogen,which is mainly due to the strong seasonal changes in the low-energy coast or shallow waters of the gulf.At the end of the Cretaceous,the source rocks of Batamayineishan Formation matured in large areas in the depression and slope areas in the southeast of Junggar Basin,and they have generally entered a high-mature to over-mature stage,while only part of source rocks in the uplifting areas have entered the main oil generation window.Volcanic activity has obvious promotion effect on the formation and evolution of source rocks:some source rocks are rich in tuffaceous components and show good hydrocarbon generation capability.In some areas,the vitrinite reflectance of source rocks is abnormally high due to heating by magma,causing its hydrocarbon generation window to migrate to the shallow part.In the southeast of Junggar Basin,the Carboniferous source rock of Batamayineishan Formation has a thickness of over 100 m,covering an area of 3835.4 km^(2).In the east slope of the Fukang sag and Jimsar sag,there are two gas generation centers with the gas generation intensity greater than 100×10^(8) m^(3)/km^(2).On the whole,the southeast of Junggar Basin has the resource potential to form large-and medium-sized gas fields,and it is expected to become an important area for natural gas exploration following the Kalameili gas field.