Main Controlling Factors Of Hydrocarbon Accumulation In The Deep Zones And Prediction Of Favorable Areas In Yanchang Oilfield

In recent years, with the advance of technical means and expansion of the exploration field, groups of reservoirs were found in the deep zones of Upper Triassic Yanchang formation. One of the most representatives is Xifeng oilfield found in southeastern of Ordos Basin with high-quality reserves of400million tons in underwater fan, and Well Gao-52was drilled in Ansai with daily output of64.7t/d. The good results were also made in formations of Chang8,Chang9and Chang10of Wuqi-Zhidan area by YanChang Oilfield. In this paper, the oil reservoirs in lower part of Yanchang formation was studied to analyze the deposition type, reservoir characteristics, feature of hydrocarbon source rocks, force of petroleum migration-accumulation and accumulation combinations, to reveal the main controlling factors of accumulation, and to summarize the rules of accumulation.The new views are as follows:1. Through stratigraphic correlation,the strata distrubution and variation rules of the study ares was knew.That is, the stratigraphic thickness of Chang101remain stable with average thickness50-70m,relativly large in northwest; the average thickness of Chang9is90-130m with the Lijiaban shales developedn in eastern part; the average thickness of Chang8is50-100m,thickening gradually from north to south and thinning from east to west; the average thickness of Chang7is70-110m, thickening gradually from north to south and thinning from west to east,then thickening again.2. The shallow-water delta was developed in the lower part of Yanchang formation, the deep lake&sublacustrine fan only found in Chang7, the sandbody in which was controlled by swing of the lake shourline and that were mainly distributed in underwater distributary channel and mouth bar microfacies of the delta front subfacies.3. The feldspar sand was mainly developed with good separation and less rounded; dominated by calcite cements except lomontite cement in Chang101-2. The type of pores is mainly primary pore and its structure is small pore with thin throat, or medium pore with thin throat. Sedimentary micro-facies and lithogenesis are the primary factors controlling and influencing the formation development and spreading of target region. According to Sonic-resistance relationship, the electrical standard of reservoir in the lower part of Yanchang formation in different regions. 4. It is presented that three sets of hydrocarbon source rocks (Chang7, Chang9and Chang10, among which the Chang7as the main hydrocarbon source rocks, mainly located in the western and southern parts of the study area; the secondary one of Chang9located in the southeastern region and the secondary one of Chang10mainly located in the central and western regions of the study area. The organic matters of Chang7source rock is I type with high maturity which belongs to excellent hydrocarbon, and the type of Chang9and Chang10is Ⅱ type, high maturity, belongs to better hydrocarbon. The source of reservoir is one-way or two-way direction, showing the accumulation model "injection migration"or"lower-generation and upper storage".5. It was showed that the overpressure and the overpressure gap of adjacent layers was the main force of oil migration. The orientation of migration is the relatively low-value area on the plane and relatively high value in the background of low-gap is the residence of hydrocarbon accumulation.6. The factors of accumulation in lower part of Yanchang formation are different, which could be divided into three types:controlling reservoir, paleostructure controlling reservoir and reservoir controlled by pressure&paleostructure. It is shown that the reservoir distributed in where hydrocarbon source developed, the delta-front and gravity-flow sediment are the main areas for the ultra-low-permeability reservoirs and the places of low-overpressure are the preferential places for hydrocarbon migration&accumulation.7. Integrated using of sand body thickness contours with hydrocarbon source rock thickness congruency map, combined with overpressure, logging and testing, the rich areas of accumulation were indicated, among which, the reserves were caculated, showing great potential of deep zones of Yanchang formation.