Genesis of authigenic clay minerals and their impacts on reservoir quality in tight conglomerate reservoirs of the Triassic Baikouquan formation in the Mahu Sag, Junggar Basin, Western China

Abstract

Authigenic clay minerals are widely distributed and considerably affect the reservoir quality of the conglomerate reservoirs in the Triassic Baikouquan Formation (BF), Mahu Sag, Junggar Basin, Western China. Their occurrences, origins, and impacts on reservoir quality were investigated utilizing petrographical analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), Mercury intrusion capillary pressure (MICP), and fractal analysis. Pore-lining chlorite coatings (Ch1) and pore-filling chlorite aggregates (Ch2), respectively, originate from smectite precursors and directly precipitate from pore water. The first-stage kaolinite (K1) formed by plagioclase dissolution appears as flake-shaped single crystals, whereas the second-stage kaolinite (K2) formed by K-feldspar particle dissolution appears as complete false-hexagonal single crystals and vermiform-shaped aggregates. Smectite transformation produces first-stage illite (I1) with honeycomb-like shaped aggregates. The second-stage illite (I2) displaying a flake-shaped occurrence with a directional arrangement is attributed to the transformation of K-feldspar. Based on the burial history, contact relationship of diagenetic minerals, and hydrocarbon charging period, the paragenetic sequence of the conglomerate reservoir is summarized.Afterward, Neimark's fractal model is employed to calculate the fractal dimensions (Fd) of the reservoir with different dominant clay minerals. It is revealed that pore-lining chlorites have the smallest Fd values (<2.5) and their pore size reached 0.6 μm, whereas pore-filling chlorites possessed the greatest heterogeneity with the narrowest pore size distribution. Illite and kaolinite affect reservoir quality in both favorable and detrimental ways. Eventually, the pore evolution within different lithofacies was analyzed. The first promising reservoirs are well-sorted fine-grained conglomerates that have experienced two phases of oil charging, wherein well-developed primary intergranular apertures, secondary dissolution apertures, and kaolinite are commonly observed. Medium-grained conglomerate reservoirs with high clay amounts and poor sorting have been subjected to rapid and intense compaction, resulting in the worst reservoir quality while no dissolution occurred. This work may provide a useful reference for hydrocarbon exploitation of reservoirs which formed under similar geological conditions.