Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Spatial Diagenetic Heterogeneity of Lenticular Sandbody in Shahejie Formation, Bohai Bay Basin, China, and Its Implications for Sandstone Diagenesis
(1) College of Geosciences, China University of Petroleum, Beijing, China.
The entity of diagenesis which refers to the physical and chemical processes that affected sedimentary materials after deposition and weathering was always an outstanding issue. We try to figure out its complicated process by studying a whole diagenetic system, which are composed of the deeply buried (3100~3300m) lenticular sandbodies and the surrounded shales. The lenticular sandbodies of five diverse distribution patterns were lying at Shahejie Formation in the rift basin of eastern China. 141 core samples, including sandstones and shales, were acquired within sandbody and near the sand/shale contact (SSC) by densely sampling. Based on analysis of thin section, X-Ray Diffraction, Scanning Electron Microscopy and Cathodoluminescence Micrography, those samples’ diagenesis and its differences between the exterior and the interior of lenticular sandbody were analyzed.
We found the lenticular sandstones got high porosity (20%) at the interior and low porosity (4%) at the SSC. The reason for low porosity was the present of carbonate cements. The quantity and sorts of carbonate cements formed during increasing burial depth is much more in the external (>15%) than in the interior of lenticular sandbody (<3%). Shale supplied the source for a portion of the diagenetic cements near the SSC in the whole diagenetic system Thin sections showed that between 65°C and 140°C, sandstones and shales underwent massive chemical and textural reorganization. In this temperature interval, within sandstones and shales of late-stage illitization, unequal amounts of reactive components in the SSC created episodic chemical gradients, so the diffusive mass transport process was formed in the whole diagenetic system. Near the SSC, a cement band was formed rapidly on a geological scale and sufficient to take up over distances of 1 to 3 meters in lenticular sandbody. Sufficient burial depth could afford the temperature threshold to the formation of chemical gradient. The scale of lenticular sandbody and shales would influence the cement extent and cement distribution. Sorts and quantity of composition in both sand and shale would dominate episodic diffusion gradients and the direction and velocity of the transfer mass. The whole diagenesis evolution result proved that shales play an important role in the process and must be considered in to sandstone diagenesis.
Key words: Lenticular sandbody; Diagenestic system