Research On Sedimentary Facies And Petroleum Accumulation Regularity Of Chang 6 And Chang 4+5 Oil-bearing Layers In Ansai Area

Being the main oil production layers of Ansai area in Ordos basin, Chang 6 and Chang 4+5 oil-bearing layers of Jurassic Yangchang Formation have been deeply studied from the level of reservoirs, while there being little research from the perspective of areas. To locate exploration targets, it is of great strategic significance to study sedimentary facies across the whole area, accurately describe Ansai delta’s geometry of the two periods and ascertain the petroleum accumulation regularity.The research selectes 840 wells to study Chang 6 and Chang 4+5 oil-bearing layers, which are the main production members in this area. Based on field profile observation, core description, drilling data, logging data and test data, by studing the formation thickness of objective strata, the stretching characteristics in plane of the thickness ratio of sandstone in strata, the structure of sandbody, sedimentary facies and the thickness of sandstone, combining the distribution characteristics of proven reservoirs, the thesis summarizes the petroleum accumulation regularity and its main controlling factors in this area.The thickness of marker beds in Ansai area wholely presenting the trend of gradually thickening from northeast to southwest, it suggests that the center of basin locates in southwest. Strata thickness suddenly thickening means the bottom of basin sloping, indirectly reflecting the location of shoreline. Ansai area was a shallow water platform delta in Chang 6 and Chang 4+5 depositional periods, which looked like a leaf, and sandstone of the two periods stretched similarly, both developing four main delta lobes and seven main sand belts, stretching from northeast to southwest. The shoreline of Chang 6 period approximately stretched along the line strip of Zhangjiayuan—Siwan—Pingqiao—Sigou—Haojiaping, and the shoreline of Chang 4+52 period approximately stretched along the line strip of Qujiapan—Caojiagua—Wangmaowan. Delta front subfacies were mainly developed all over Ansai area, meanwhile delta plain subfacies being developed in the northeast.Reservoirs of Ansai area distribute unevenly. There are two petroleum accumulation regularities in this area. (1) The interior sand belts of deltaic front subaqueous distributary channel control the distribution of high abundance multi-reservoirs; (2) Petroleum compensatorily accumulates as the way of ladder-like rising along main sand belts and oil-bearing layers being upward progressively new. The distribution of reservoirs is mainly affected by (1) main sand belts of underwater distributary channel (or distributary channel), (2) lateral occlusions including mudstone of distributary bay or tight sandstone of channel flank, (3) the distribution of dissolution pores including laumontite dissoluted pores and feldspar dissoluted pore, while nose-like uplift working in partial regions. By summarizing the above results, the research predicts the following three favorable areas:southwest of Zhenwudong district, southwest of Jianhuasi district, north of Tanjiaying district.