Sedimentology And Reservoir Research Of The Chang8Oil Layer In Yanchang Formation Of Triassic,Jiyuan Area, Erdos Basin

Jiyuan area located in the middle of Tianhuan Depression, which was in the middle and west regions of Ordos Basin. Chang8reservoirs, whose sedimentary facies was lacustrian-delta deposits system, was an important producing layer, with lithological traps as the main reservoir. With the increase of the production and exploit degree, it was needed to enhance the recognization on the reservoir characteristics. But the reservoir property was poor, such as the sandstone was in low porosity-permeability and high cementation and anisotropy, also the study area lacked of research on the diagenesis and reservoir features. Facing with that situation, the depositional facies and reservoir characteristics were studied and discussed deeply by analysis on cores and logging data, thin sections identification, cathodoluminescence images, scanning electron microscopic images and vitrinite reflectance, on the guidance of sedimentology and petroleum geological theory. The research obtained some understandings as follows:On basis of large amount of careful core observation,21lithofacies were identified, such as sandstone with trough cross stratification(St), sandstone with tabular cross bedding(Sp), Sandstone with parallel bedding(Sh), sandstone with Ripple bedding(Sr), siltstone with ripple bedding(Fr), siltstone with climbing ripple bedding(Fc), siltstone with lenticular bedding(Fl), and so on. And there were two kinds of most commonly lithofacies associations in the study area, i.e sandstone association in positive rhythm and association of dark mudstone combined-silty sandstone. The former was usually occurring in the distributary channel and underwater distributary channel, while the latter appeared more often in the underwater levee.Also the logging curves analysis showed that, box and bell shape curves were the characteristics of distributary channel and underwater distributary channel, which were rich in sandstone, the GR curve of levee showed finger shape, but crevasse splay was funnel shape, and the smooth-microfold or lineral curves corresponded to underwater interdistributary bay.The blanket delta was developed in the reservoirs of Chang8. And the facies in the study area were mainly delta plain and delta front, distributary channel, interdistributary, levee and crevasse splay were the main microfacies in delta plain, and underwater distributary channel, tributary bay and underwater levee and underwater crevasse splay. Among those facies, underwater distributary channel was the beneficial belt for oil and gas accumulation.Petrologic feature of reservoir showed that types of rock were arkoses and feldspathic lithic sandstone, in which the content of feldspar was between42.78%and52.16%, average in48.75%, and quartz accounted for21.99to29.18%, average in25.12%. The interstitial material included chlorite, calcite, ferrocalcite, goeschwitzite and kiesel. And the cementing matter consisted of carbonate, kaolinite, kiesel and chlorite. The grain of the sandstone was fine, sub-angular, but sorting medium-good. In sum, sandstone of study area had low compositional maturity but high textural maturity.Diagenesis study signified that the main types of diagenesis included compaction (pressure solution), cementation, corrosion and replacement. Among those, compaction (pressure solution) and cementation were the important contributors to the low porosity and low permeability. The secondary porosity generated by the dissolution of the feldspar mean a lot to the reservoir. And the reservoir had been classified as A eriod of eogenetic phase, locally was B eriod of middle diagenesis phase.On this basis, the reservoir characteristics were studied by the collection of porosity and permeability data. The result showed that Chang8was typical low porosity-permeability reservoir. And the porosity was between7.49%and14.39%, average in10.79%, on the other hand, the permeability distributed at the range of0.08～4.79×10-3μm2, average in1.15×10-3μm2.The pore structure and reservior properties were analyzed on the start of petrologic characteristics of reservoir. The controlling factors of original mineral composition, diagenesis and heterogeneity on the quality of reservoir were stated,moreover the diagenesis facies was classified and the distributional regularity was pointed out. In all, the diagenetic facies of Chang8in Jiyuan area were ferroan calcite cementation facies, chlorite-arkose denudation facies and kaolinite-ferroan calcite cementation facies.The standards for reservoir comprehensive evaluation was set up in the study area, and three types of reservoirs from type I to type III were classified. And type I and type II distributed in the overlapped area of underwater distributary channels, which lied in the northwest and middle of study area, while type III lied in the flanks of underwater distributary channels.