The Oil Reservoir Forming Mechanisms And Its Main Controlling Factors Of The Lower Yanchang Formation In The Middle Of The West Ordos Basin

The lower Yanchang formation was a new exploration area in the middle of the west Ordos basin. The reservoirs were the injection migration type, but the oil reservoir forming mechanism and its main controlling factors were unclear. The solution of the problems not only direct to the oil exploration but also with scientific meanings.The research area sedimentary was controlled by the northwest sedimentary system which came from Alasan according to the light, heavy minerals, detritus assembles and the paleo-flow directions. The Chang9 oil set mainly developed delta sedimentary and semi-deep lake facies according to the outcrops, cores, well loggings data et al. The Chang 8 oil set developed shallow water delta and the sands were controlled by the palace of the lake strandline. The sand of Chang8 oil set with the characters of local thick sands pod distributed along the river and ringed distributed crosscut the river. The sands distributed characters were controlled by the position of the lake strandlines, that were lake strandline control sands.The late digenesis was the main factor which controlled the reservoir qualities. Ferroan calcite cement led the reservoir poor and the chlorite velum developed formed local high quality reservoirs. Three diagenetic facies were divided after research. The chlorite velum+ kaolinite cement was the best diagenetic facies and the chlorite velum+ ferroan calcite cement was the super best diagenetic facies.The high-class source rock of Chang7 in Ordos Basin increasing pressure action from hydrocarbon generation expanding which was the main dynamic force for the oil migration down to the lower reservoirs. The lower pressure area or the relative lower pressure areas were the favorite areas for the oil accumulation. The Chang8 and Chang9 reservoirs were the same play that was under source play with the "continuous accumulation, episodic charging" characters according to the over pressure, hydraulic fracturing, boiling oil and gas inclusions, homogenization temperature analyzed. There are four oil accumulation types in the research area, that were reservoir contact the source directory style, from the hydraulic fractures accumulation style, from the penetrative sands accumulation style and the from the hydraulic fractures penetrative sands accumulation style.The reservoirs formation and distribution were controlled bye the "near source , lower pressure, fine facies" factors of the lower Yanchang formation in the middle of the west Ordos basin and 7 targets were deleted in Chang8 oil set.