Diagenesis and hydrocarbon charging of the tight-sand reservoir in the Upper Triassic Chang-8 Member, Ordos Basin: Implications for porosity evolution

The Upper Triassic Chang-8 Member of Zhenjing district, Ordos basin, distinguished by widely distributed shallow-water delta-front deposits, is currently a hydrocarbon exploration target. The study investigates the influence... [ view full abstract ]

The Upper Triassic Chang-8 Member of Zhenjing district, Ordos basin, distinguished by widely distributed shallow-water delta-front deposits, is currently a hydrocarbon exploration target. The study investigates the influence of the coupling relationship between diagenetic evolution and oil charging process on porosity evolution of tight-sand reservoir.

Here we document data from mineralogical and diagenetic observations of 192 samples from 28 wells, including cathode luminescence thin sections, fluorescence thin sections, SEM, XRD analysis, carbon-oxygen isotope analysis, hydrocarbon inclusions and K-Ar isotopic dating of authigenic illite. These analyses indicate a complex diagenetic sequence accompanied with two-phase of hydrocarbon charging. The compaction and cementation are passive diagenesis for reservoir quality, resulting in decreases of porosity by 14.6% and 22.6%, respectively. Based on the volume of different cement types, the reduction of reservoir porosity caused by cementation can be further subdivided into 15% by calcite, 3.3% by clay minerals, and 2.3% by Silica. Geochemical analyses suggest that the mean δ13C and δ18O isotopic compositions of the intergranular calcite are -4.1 ± 1.3‰, VPDB and -19.8 ± 1.2‰, VPDB, respectively. Microscopic analysis shows that the dominant cement is intergranular calcite which formed extensively after corrosion of feldspar and was followed by the large scale of hydrocarbon inpouring. The two-phase fluid inclusions were identified by the rims of quartz overgrowths. The porosity of the Chang-8 reservoir remained at 15%~20% in the earlier main period of accumulation (143Ma~121Ma). During the later main period (121Ma~100Ma), the calcite cementation drived the reservoir to enter the densification stage. Reservoirs with high oil saturation maintain higher ratio of primary pores to secondary pores than those with low oil saturation.

The combined evidences from petrographic, microthermometric and geochemical analyses suggest that (1) the calcite cementation derived from the oxidation of organic matters acts as a main controlling factor on the porosity evolution of tight-sand reservoir in the Chang-8 Member; (2) the accumulation of hydrocarbon can inhibit diagenesis, especially cementation and dissolution, whereas the degree of inhibition depends on oil saturation of reservoirs.