Sedimentary Facies And Petroleum Accumulation Regularity Of Chang 2 Oil-bearing Layer In AnSai Oilfield

Located in the north of central and eastern North Shaanxi Slope, Ansai Oil Production Plant ranks among the important energy reservoirs of the Ordos Basin. Currently Xingzichuan Oil Production Plant is at the middle and later periods of oilfield development, and so far great progress has been made on reservoir characterizations in Haojiaping, Wangjiawan, Tanjiaying and other important oil pools in Ansai areas. However, unified knowledge on overall sedimentary facies and other basic geological conditions in Ansai areas is yet to be established. To expand exploration ranges, locate exploration targets and reinforce exploration work, it is necessary and practically significant to conduct basic geological studies on sedimentary facies across the whole areas, have a clear idea of the geometric structure of Ansai Delta, and ascertain the oil distribution and aggregation laws.The project focused on the key oil-bearing formation Chang 2 oil pool of Ansai area. Guided by multidisciplinary principles like sedimentology, petroleum geology, geophysics and so on, the paper systematically studied stratum structure features, laws on sedimentary microfacies classification and development, laws on sand body distribution and oil reservoir distribution laws in the area, relying on regional information investigations, field core descriptions, observations of facies markers, and analysis and explanations of logging data. Finally the paper analyzed the relations between hydrocarbon accumulation and aforesaid geological conditions, probed into the distribution laws and major controlling factors of Chang 2 oil pool, and ascertained advantageous conditions for further exploration of Xingzichuan oil field.Studies on the stratigraphic classification and correlations revealed that the K9 marker beds at the top of Chang 21 are common in the studied areas. The total stratum thickness of Chang 2 range from 120 m to 150 m, and southern areas of the oil pool feature thicker stratum than the northern ones. Thickness range of various single layers is shown as follows:5?10m with Chang 211, the smallest in thickness; 10-15m with Chang 212; 25-35m with Chang 213, the largest in thickness. Various sand formations are stable and consistent in thickness. According to superface structural features of the sand formations, there are 5 lines of relatively sharp nose-shaped uplifts running from east to west in the northern areas. And there are 3 lines of slightly nose-like structures running from east to west in the central and southern areas, comparatively wider, flatter and more open than the northern areas.The Ansai areas, where the Chang 2 oil pool is, fall into the Ansai-Yan'an Delta depositional system subject to the northeast provenances. After analysis of the sedimentation granularity characters, electrofacies and sedimentary structures, the paper argued that the Chang 21 stratigraphical interval main bodies fell into deltaic plain subfacies sedimentation and could be further classified into distributary channel, distributary channel side vane, flood fan, and peat bog.Based on fine industrial mapping, horizontal distribution laws of Ansai Delta were made clear:three main deltaic lobes and seven main sand belts. The northern Wangjiawan Liandaowan main lobes belong to Wushenqi-Jingbian Delta. The Pingqiao-Huaziping main lobes and Haojiaping-Zhao'an lobes belong to AnSai Delta. The Yanhewan-Gaoqiao lobes belong to Yan'an Delta,Chang 2~1 is an important pay bed of the studied areas, with oil reservoirs distributed mainly in Haojiaping areas, Wangjiawan areas and Meita areas. The oil reservoirs are in planar zonal distribution along the main distributary channels. Oil reservoir is subject mainly to sedimentary facies and the development level of sand bodies. The nose-like structure is an important influential factor to Chang 2 oil reservoir in Ansai areas. The inner argillaceous sediment of Chang 1 and Chang21 serves as favorable cap rocks and lateral barriers, thus forming the unique oil and gas accumulation model-ladder-like climbing migration plus structural ridges.