The Rodessa Limestone (Aptian) produces from structural and combination traps in the East Texas basin. West Purt field, in northeastern Anderson County, is a Rodessa combination trap where porosity and permeability have been affected by hydrocarbon alteration, adding an additional complexity to the reservoir.

In West Purt field, porous skeletal grainstones successively pinch out obliquely across the crest of a northwest-plunging structural nose. The structure is cut by a eastward-dipping fault that forms the eastern boundary of the field. The reservoir grainstones have been subdivided into 3 facies. Two of these facies are fine-grained to cobble-size, poorly sorted coral-skeletal rudstone, cyclicly interbedded with fine-grained, well-sorted mollusk-echinoid grainstone and packstone. The third facies is the overlying fine to coarse-grained mollusk-peloid grainstone, commonly laminated or graded. The overall sequence is interpreted as a prograding shoreface and foreshore deposit.

Among the more significant aspects of diagenesis are the early formation of moldic porosity that is partially filled with phreatic isopachous and equant calcite spar cements. Later compaction and minor cementation by saddle dolomite and anhydrite had a minimal effect on porosity. The final stage of cementation was the precipitation of solid bitumen. This bitumen causes a moderate decrease in core-measured porosity, but a significant decrease in permeability by plugging pore throats. The presence and distribution of solid bitumen are not discernible on logs owing to the lack of significant density contrast between crude oil and bitumen. Solid bitumen occurs only in wells adjacent to the eastern boundary fault, regardless of structural elevation. Geochemical analyses of bitumen samples uggest that secondary gas from an underlying source (migrating up the eastern boundary fault) caused the precipitation of solid bitumen by deasphalting the in-place oil.

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