内蒙古河套盆地白垩纪古气候、沉积环境及油气勘探远景

CRETACEOUS PALAEOCLIMATE,SEDIMENTARY ENVIRONMENT AND OIL-GAS PROSPECT IN THE HETAO BASIN, INNER MONGOLIA

河套盆地陆相白垩系分为乌尔塔组、大水沟组、乌兰呼少组、毕克齐组和杭锦组。早、中白垩世河套盆地与陕甘宁盆地连通,晚白垩世分隔形成独立的封闭型山前断陷盆地。该套地层石膏质泥岩发育,耐旱性孢粉占优势,显示了当时为干旱和半干旱亚热带气候。沉积相由山麓相、河流相过渡为滨—浅湖相和半深水湖相,在大水沟组和杭锦组还见到生油岩组合。

The Hetao Basin in Inner Mongolia is a Mesozoic-Cenozoic piedmont downfaulted basin with the lower and middle precambrian granite-gneiss area of about 40000km^2. There the Cretaceous System is well developed and may be divided in ascending order into the Wuerta Formation (K_1w), Dashuigou Formation (K_2d), Ulan Hushao Formation (K_2w), Bikeqi Formation (K_2d) and Hangjin Formation (K_3h). There are two sequences of oil source rocks in the basin: one occurs in the Dashuigou Formation, and the other in the Hangjin Formation. According to the succession of the strata, the formation and evolution of the basin are the same as those of the Songliao Basin and Erlian Basin. Its palaeoclimate, sedimentary environment, sedimentary types and organic matter were controlled by the Inner Mongolian axis, and the materials were derived from granite-gneiss in the Langshan and Daqing Mountains north of it. During the Early and Middle Cretaceous, the Hetao Basin was linked with the Shan-Gan-Ning Basia and Guyang Basin, but in the Late Cretaceous, it was separated from the latters to form an independent closed piedmont down-faulted basin. Gypseous mudstones is developed and drought-enduring sporo-pollen-predominates, indicating an arid and semiarid subtropical palaeoclimate. Sedimentary facies changes from piedmont facies and fluvial facies to lake shore-shallow lake facies and semideep lake facies. The oxidation is strong. Owing to thin vegetation and high salinity of the lake water, the basin was not suitable for the reproduction of aquatic organisms, The thickness of the oil-source rooks and abundance of residual, organic matter are less then those in the Songliao Basin and Erlian Basin.