坳陷湖盆三角洲前缘储层砂体成因研究

GENETIC RESEARCH OF RESERVOIRS AT DELTA FRONT IN DEPRESSED LACUSTRINE BASINS

晚三叠世鄂尔多斯盆地是一个东缓西陡的不对称坳陷盆地,延长组主要油气资源分布在东部缓坡带。运用高分辨率层序地层学的短期基准面旋回原理,对发育于姬塬—安边地区长6油层组三角洲前缘的储集砂体进行细致分析,按成因将储层砂体详细划分为3大类7种类型;该砂体受控于基准面升降、可容纳空间和沉积物补给量之间的变化关系2个方面。基准面升降过程中,短期基准面旋回的规律性变化影响着不同成因砂体的分布,据此建立了坳陷湖盆三角洲前缘不同砂体类型的成因分布模式。以坡折带为界,将研究区三角洲前缘划分为“台型前缘”、“坡型前缘”和“盆型前缘”3个区带,各区带发育不同储层砂体成因类型:“台型前缘”区主要发育截削式河道砂体和完整式河道砂体,“坡型前缘”区主要发育“下残式坝上河”,“盆型前缘”区主要发育完整式、上残式坝上河砂体和完整式河口坝砂体。这种对三角洲前缘的详细划分以及对储层砂体的精细描述有助于优质储层的鉴别和分布预测,对油藏开发中流动单元的细分也有所帮助。

During the Late Triassic, the Ordos Basin is an asymmetry depressed lacustrine basin with gentle slope in the east and steep slope in the west. In the Yanchang Formation, oil is mainly distributed at the gentle slope in the east. With the short-term base-level cycle theory of high-resolution sequence stratigraphy, it is analized in this paper the reservoir sandstones deposited at delta front in the Chang Ⅵ oil formation in Jiyuan and Anbian,the Ordos Basin. According to genetic difference, reservoir sandstones may be divided into three types and seven subtypes, evidently influenced by the short-term base-level up and down changes and the relation between accommodations and sediment alimentations. As the base-level goes up and down, regular changes of short-term base-level cycle affect the distribution of different genetic sandstone bodies. And the distribution model of different genetic sandstones deposited at delta front in depressed lacustrine basins is built. The delta front in this area can be divided into three parts： platform delta front, sloping delta front and basin delta front by the bounds of slope break. Reservoir sandstones are genetically different at different parts. Truncated and integrated channel sandstone bodies are found at platform delta front. Downside deformity recombination bar sandstone bodies are found at sloping delta front. Integral recombination bar, upside deformity recombination bar, isolated mouth bar and reiterative mouth bar sandstone bodies are found at basin delta front. The compartment scheme of delta front and the discription of reservoir bodies can be helpful for high quality reservoir identification and prediction, and it can also serve to subdivide flow units during oil exploitation.