上扬子地区寒武纪岩相古地理：对中国小陆块海相盆地演化特点及其控藏效应的启示

Cambrian lithofacies paleogeographic characteristics of the Upper Yangtze Block: implications for the marine basin evolution and hydrocarbon accumulation of small-scale tectonic blocks in China

关键词:  古地理重建  碳酸盐岩  深层油气  寒武纪  上扬子板块

Key words:  Paleogeographic reconstruction  Carbonate  Deep nature gas  Cambrian  Upper Yangtze Block

摘要:

岩相古地理重建对认识盆地的沉积环境演化、沉积建造时空分布和油气勘探具有重要意义。为揭示上扬子地区寒武纪古地理背景这一先决条件对新发现的安岳巨型气田的控制作用及其对碳酸盐岩油气勘探的启示，本文综合油气勘探的地震、钻井资料及盆地周缘的露头剖面地质调查成果，重建了寒武纪两个构造层序的岩相古地理，分析了寒武系生储盖组合发育的特征。结果表明：早寒武世早期，伸展背景形成了破碎型克拉通盆地，近南北向的深水槽切割了上扬子克拉通，充填了丰富的富有机质沉积物，成为深层巨型生烃灶；早寒武世晚期—晚寒武世，上扬子板块转变为准稳定的构造背景，盆地性质转换为陆表海型克拉通坳陷盆地，沉积建造以发育潮坪楔及伴生的膏盐为特征，陆表海潮汐作用和古隆起控制了储层宏观分布；构造活跃期的裂陷沉积之上叠加以准稳定期的克拉通内坳陷沉积，形成了类似裂陷盆地的由裂谷层序和后裂谷层序组成的剖面形态像牛头的牛头型沉积建造结构；与大板块若即若离的亲缘过程中，构造活跃期和构造准稳定期交替叠合的盆地古地理特点是我国前晚三叠世小陆块离散演化阶段的一大特色，在沉积建造上形成良好的生-储-盖空间配置，是克拉通油气勘探的两大重要领域。

Abstract:

The reconstruction of litho-paleogeography plays an important role in understanding the evolution of sedimentary environment, spatial distribution of sedimentary formations and hydrocarbon exploration of basins. In order to reveal the Cambrian paleogeographic setting which is the controlling precondition of the newly discovered Anyue giant gas field in the Upper Yangtze Block and its significances to potential carbonate oil and gas exploration, based on the seismic and borehole-logging data and the geological survey of outcrop section surrounding the Sichuan Basin, the lithofacies and paleogeography of the two Cambrian tectonic sequences were reconstructed, and the characteristics of Cambrian source-reservoir-seal assemblage were analyzed. The results showed that, in the earlier Eearly Cambrian, the tectonic extensional background induced a broken craton basin which had a deep trough cutting the Upper Yangtze Block with north-south direction.The infill of organic-rich sediments in the trough became the kitchen of deep giant hydrocarbonlayers. From the later Early Cambrian to the late Cambrian, the upper Yangtze plate shifted to a relatively stable tectonic setting, and the basins on the Upper Yangtze Block were unified and formed a craton depression basin. The sedimentary formation was characterized by the development of the epeiric tidal wedge and associated salt. The tidal energy in the epeiric sea constrained the macroscopic distribution of the reservoirs. The deposits filling in tectonic active stages were superimposed by infill of the relatively stable intracraton depressions, forming a sequence combination similar to the rift basin which is composed of rift and post-rift sequences. The transection-profile of this type of sequence combination is like a cartoon of the ox-head. The paleogeographic characteristics of the basin, which experienced tectonic active interval and tectonic stability interval alternately, was a major feature of Chinese small-scale blocks linked to their affinity with the big-scale plates and drifting discretely in paleo-oceans in the pre-Late Triassic period. This special was favor for forming potential source-reservoir-seal assemblages in the sedimentary formation which can develop giant hydrocarbon reservoir in intracratonic basins.