Diagenesis of Triassic Sandstones in the Taylorsville Basin of Virginia

Guoqiu Gao, Tullis C. Onstott, Hsin-Yi. Tseng

The Taylorsville Basin in Virginia is one of the syn-rift basins in eastern North America which formed as a result of the breakup of Pangea during the Triassic-Jurassic time. Although there are as many as twenty syn-rift basins and many of these basins are being explored for hydrocarbons, sandstone diagenesis and its effects on reservoir quality in these non-marine basins are still poorly understood. To advance our understanding of sandstone diagenesis in the syn-rift basins, we have been conducting petrographic and geochemical investigations on the diagenesis of sandstones from the Triassic Doswell Formation of the Taylorsville Basin.

Triassic sandstones in the Taylorsville Basin were deposited during late Triassic time in alluvial fan, fluvial, and lacustrine settings. Preliminary work on cores from three wells and on cuttings from one well demonstrates that the Triassic sandstones, mostly arkoses and lithic arkoses in framework grain compositions, have undergone significant diagenetic modifications. These include (1) extensive compaction through grain rearrangement, ductile deformation of soft grains such as micas and clay-rich rock fragments, and minor pressure solution of quartz grains, (2) cementation by quartz, clay minerals (mainly chlorite and illite), calcite, laumontite, and minor feldspar and pyrite, (3) albitization of detrital plagioclase and K-feldspar, and (4) dissolution of detrital feldspars, carbo ates, and rock fragments.

The reservoir quality of the Triassic sandstones in the Taylorsville Basin is controlled both by deposition fabrics and diagenesis. Sandstone samples with poor sorting and abundant ductile grains usually have low primary porosity. Post-depositional porosity reduction by mechanical compaction and cementation is significant, although minor secondary porosity was generated through dissolution of detrital components.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995