Abstract:

Many lenticular sandbodies developed in dark shales of Shahejie Formation in Zhanhua depression. With analysis of cast thin section, X-ray diffraction, scanning electron microscopy， fluorescent light and cathodoluminescence micrography, diagenetic differences from the exterior to the interior of the typical lenticular sandbody were analyzed by dense sampling from 3 199 m to 3 201 m. There was intense dissolution in the sandstones, but carbonate cementation, quartz overgrowth and feldspar overgrowth decreased from the exterior to the interior of sandbody. The later carbonate cement formed a tight cemented layer, which decreased the sandstones porosity. However, the oil and gas accumulation was not impacted by the tight cemented layer. By analysing the sequence of diagenesis and the oil accumulation, we find the cementation was later than the oil accumulation. The secondary porosity of the sandbody showed the early intense dissolution, which supplied enough space for the hydrocarbon charging peak and decreased the capillary resistant force. After the oil and gas accumulation, the later carbonate cement decreased the exterior sandstones porosity and increased the capillary resistant force, which depressed the oil charge and protected the reservoir. In a word, not all tight cemented layers near the exterior sandbody could disturb the oil and gas accumulation.

Key words: Zhanhua depression, lenticular sandbody, diagenesis, petroleum gas