| Ordos Basin, which is rich in mineral resources, is the second largest sedimentary basin in China. It provides large quantities of oil and natural gas resources to Changqing oilfield every year. Zhenbei-Huanxian Area is located in the southwest of Ordos Basin. It starts from Huanxian in the north, to Chongxin in the south, to Pengyang in the west, and to Taibailiang in the east. The land covers60-90km from east to west, while covers190km from north to south, with a total area of1.5×104km2. Its administrative region belongs to Pengyang County in Ningxia, Qingcheng Country, Zhenyuan Country, southern Huanxian, and western Xifeng in Gansu. The surface belongs to loess geomorphology and the elevation of the land is1140-1400m, with the relative height difference of about260m. In structure, it includes two great tectonic units, which are Tianhuan Depression and Yishan Slope.Tianhuan depression, which is located in the west of Ordos Basin, is the junction between the overthrust belt on western border of Ordos Basin and the Yishan Slope. It covers50-60km from east to west, while covers600km from north to south, with a total area of about32x104km2. Tectonic features is a gently west inclined slope (with an angle of less than1°), inserting under the thrust belt as subduction. Yishan Slope is the largest tectonic unit in Ordos Basin. Late Proterozoic and early Paleozoic are uplift areas. There are no sediments deposited. Only in the middle-late Cambrian and early Ordovician, there are marine strata with the total sedimentary thickness of about500-1000m.Wuqi-Dingbian-Qingyang are the paleouplifts with sedimentary thickness which is only250m. After the late Paleozoic, continental sedimentary was undertaken. The current Yishan Slope, which is the main tectonic unit of Ordos Basin, was formed in the early Cretaceous, with gently westward-dipping monoclinic in general of an angle less than1°.At the beginning of the70’s, in Ordos Basin, several great Jurassic Paleo-reservoirs were discovered such as Maling, Huachi, and Wuqi, followed with several large Triassic delta lithology oil reservoirs of Yanchang formation such as Maling, Jing’an, and Huachi.The major type of reservoirs is large area, low permeability, and low abundance lithologic reservoirs. At present, the Triassic oil reserves occupy more than70%of the total oil reserves in Changqing Oilfield.Ordos Basin traditional petroleum exploration and development target strata includes three major oil reservoir sets, which are Triassic Yanchang Formation of Chang6-Chang4+5, Oil reservoir of Chang2, and Jurassic Yan’an Formation. Recent years, petroleum exploration of Chang7-Chang8oil reservoir set achieves new breakthroughs. Petroleum geological reserves at all levels increase by billion tons annually. However, Chang8oil reservoir set located at Zhenbei-Huanxian Area only explored Zhenbei Oilfield and several individual oil points with a relatively low exploration and research degree.Therefore, we analyze the controlling factors of reservoirs by anatomizing known oil and gas reservoirs. Based on this, we analyze the process of reservoir formation to clarify the major control factors of the formation. Then we try to reveal the rules of petroleum accumulation of Chang8oil reservoir set, summarize the accumulation mode, and predict the favorable areas for oil and gas distribution, to broaden the field of Mesozoic petroleum exploration in Ordos Basin.The main ideas and research contents are as follows:we summarize the target areas’ regional tectonic evolution by using the existing data and analyze current tectonic characteristics according to the drilling, logging and earthquake data of Ordos Basin. By sampling, we analyze the abundance and maturity of hydrocarbon source rocks. By applying the geologic data, logging data, and analysis assay data of known reservoirs, we depict the source rocks thickness distribution in the work area and analyze the features of distribution. We summarize the characteristics of reservoir sand body distribution and analyze the impact of diagenesis to reservoir. We also analyze the effect of reservoir physical property on the reservoir formation. According to the geologic data, logging data, and analysis assay data of the known reservoirs in Chang6-Chang8area, we anatomize and study the known reservoirs in details from the aspects of oil and gas source, migration pathway, reservoir cover characteristics, fluid property, the closure causes, the reservoir types and so on, to conclude the characteristics of reservoir. Based on the macroscopic geological factors of closure formation, combing with the comprehensive reservoir formation control factors analysis of proven reservoir, failed wells and oil wells, we analyze the main controlling factors of hydrocarbon accumulation.During the study of the project, we gather and arrange drilling data and logging data of280wells, as well as30earthquake seismic lines. The results and conclusions are as follows:(1) Based on the regional tectonic evolution, we analyze the characteristics of current tectonic distribution, realizing that in the east-west direction of work area, the current west dipping slope, which is from east to west, and depression, which is from south to north, have clear structure characteristics, and that in the south-north direction, there are southern area, middle area, and northern area, according to the different tectonic characteristics. The northern area, which includes Yinjiacheng and the north of Yanwu, mainly shows as a great depression with the center of Huan20well. The middle area, which includes the south of Yanwu and the north of Wugou, mainly shows as gently flat-bottomed land. The southern area, which includes the south of Wugou, mainly shows as a south dipping slope. Based on the analysis of the Triassic top surface tectonic graphs, we can tell that, the local structures are mainly distributed in the eastern area, with an anticline and three nose structures. While many of the Triassic oil wells are located on the nose structures, it can be seen that the hydrocarbon accumulation in the target area is impacted by tectonic conditions.(2) By precisely analyzing hydrocarbon source rocks, we know that the main hydrocarbon source rocks in this area are Triassic Yanchang formation lacustrine mudstone. According to the recovery of typical well burial history and thermal evolution history in the work area, we reach the conclusion that the mature time of hydrocarbon source rock is Early Cretaceous period. Then based on the logging data and analysis assay data, we got the thickness of source rocks and analyze the characteristics of its planar distribution. Combining the range of source rocks distribution with oil wells, it’s easy to find out that oil wells are located in the areas with thick source rocks. Therefore, accessible oil resources are the key factors of reservoir formation.(3) By analyzing reservoir physical property, diagenesis and reservoir sand body distribution, we recognize that the reservoirs of Ordos Basin Yanchang formation are especially low permeability reservoirs and super-low permeability reservoirs, while a few of them are generally low permeability reservoirs. The range of reservoir sand body distribution and the thickness of sand body have an indispensable effect on petroleum accumulation.(4) Based on the existing data, we anatomize the Zhenbei reservoir, explore the relationship between the oil and gas distribution and tectonic, sedimentary facies, sand body, and reservoirs, and conclude the petroleum accumulation rules of Zhenbei reservoir. The rules are as folloews:Zhenbei Area is near the Chang7developed area of hydrocarbon source rock with quality resource rocks. Chang7-3high gamma mudstones, which are rich in organic matters, are widely distributed in the east of target area and constitute the oil source of lots Triassic oil field in this area. The reservoir is located in the delta sedimentary system, with underwater branch channel sand body and channel mouth bar sands constitute the main reservoir. Because most of the Zhenbei reservoir is low permeability and super-low permeability sandstone, the requirement to the cover is not very harsh. For the matter of closure, due to low permeability, the filling process of oil is mandatory. If there is no subsequent injection event, the front of oil injection will remain at the last filling position, which is the reservoir boundary. Due to low permeability, capillary resistance is great. After injection reservoir, oil and formation water can not be separated and low oil saturation is formed. Most of migration pathways are by cracks.(5) In total, we analyze and sum up the main control factors of reservoir formation in the target area from the aspects of hydrocarbon source rocks, tectonic, reservoir, and so on. The conclusions are as follows:accessible oil resources are the key factor of large oil-bearing area. Quality reservoir decides the advantages of hydrocarbon accumulation. Anticline and nose structure control the oil and gas distribution of Yanchang formation. Regional tectonic lithology dominates the distribution of oil and gas. |
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