The Influence To Development Of Hydrocarbon Reservoirs From Deposition, Sedimentation And Passage System In Yanchang Formation In Ordos Basin

In Ordos Basin, the major factors to control hydrocarbon reservoirs forming and distributing are the deposition, sedimentation, uplift and erosion of source rocks in yanchang formation and sand bodies or faults connected to the main source rocks, but they were studied less. In this article, it is studied about the change of strata thickness, sedimentary facies, erosion thickness and restored thickness of chief source rocks and stratum above. Besides, it is also about the burial depth changing and thickness variating in different geologic times and the distribution of oil pools discovered already in stratum above of chief source rocks, and the interrelation about sedimentary facies belts in different strata series and congruence area of chief sand bodies. On this basis, it is discussed about the characteristics of hydrocarbon reservoirs in yanchang formation in Ordos Basin. And the following conclusions could be drawn.1) In the settling process in Ordos Basin, Yanchang formation settled deepest in the late Early Cretaceou. The depocenter located near Dingbian and Wuqi. In those sites, the maximum subsidence depth of the depocenter in Dingbian was3100m, and that in Wuqi was up to3300m. Since the Late Cretaceous, the basin uplifted and denudated. In this context, the weakest denudation was in the areas of Tiekesu Temple and Huan County-Zhenyuan with the depth about200～400m, and that in east was thicker than west, in Yonghe-Yichuan-Jixian area denudation thickness could be able to be up to1800m. Nowadays, the maximum thickness of overlying strata of Yanchang formation main Hydrocarbon source rocks is located in Jiyuan-Majiashan-mountain area, depth of approximately2700m, and800m in the basin edge.2) Yanchang formation main Hydrocarbon source rocks in Ordos Basin are mainly distributed in the Chang7oil layer group, widely distributed to the north side of Dingbian area, south to the Rich County-Qingyang, west to the Huan County west region and east to Zichang-Yanchang; hydrocarbon source rock thickness is generally3～100m, Ro evolution degree is generally0.76%to1.1%. Distribution area of main source rocks has the burial depth about2400m in the late Early Cretaceous, erosion thickness since the Late Cretaceous is generally300～1800m.3) The already found oil pools in Yanchang and overlying Yan’an Formation are mainly in the northern Shaanxi and eastern Gansu area. Intervals below Yanchang reservoir Chang7oil layer group are mainly distributed in eastern Gansu region, north of Huan County and western Ansai area, and those above it are mainly distributed in northern Shaanxi. Oil pools below Yanchang reservoir Chang7oil layer have the thicknesses about generally10～30m; those in Chang6-Chang1oil layer groups are10to80meters.4) The distribution of pathway system impacts hydrocarbon accumulation areas. Advantage channel sand bodies and cracks act as the migration pathway system of hydrocarbons from Yanchang formation, performancing of mainly cracks are predominant factors in the lower part of the group, while the upper mainly sand are predominant factors. In the overall they performance as integral contact and locally connected, the distribution of these reservoirs are located in the surrounding conducting systems. In northern Shaanxi region showed ladder-like distribution. Hydrocarbons in Yan’an group pathway system is formed by channel deposit of Yan’an group and undercut plane of Yanchang formation.5)The main hydrocarbon accumulations out of the main hydrocarbon source rock area of Yanchang formation in basin (thickness greater than30m) mainly are Chang6oil layer group reservoirs in northern Shaanxi region, and those in Chang2oil layer group, which the foundation of the former might be mainly concerned with the cracks, and that of the latter is through the advantages of sand up the ladder of fortune, with the distal lateral migration of long-distance characteristics. Reservoir zones within the main hydrocarbon source rocks are mainly in Chang6oil layer group, eastern Gansu area of Chang8oil layer group and the formation of them mainly concerned with the cracks, with near-source reservoir characteristics.