Cementation and Porosimetry of Shoaling Sequences in the Subsurface Pettit Limestone, Cretaceous of East Texas
William D. Wiggins, Paul M. Harris, 1984. "Cementation and Porosimetry of Shoaling Sequences in the Subsurface Pettit Limestone, Cretaceous of East Texas", Carbonate Sands-A Core Workshop, Paul M. Harris
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Mercury-injection porosimetry coupled with standard microscope and SEM observations have been used to study the effects of diagenetic cementation on reservoir quality in the Pettit (Sligo) interval of the W. G. Sexton No. 34 well from Western Rusk County, Texas. A 36 m (117 ft) cored interval contains three shoaling shelf limestone sequences. Each sequence is capped by coarsening-upward ooid-skeletal grainstones and has lower portions composed of packstones and wackestones with coated grains, pellets, oncolites and oyster fragments. Porosity and permeability of the grainstones decrease upward as the percentage of carbonate cement increases from a maximum 13 percent and 6 md in the lower and middle portions to 2 percent and 0.01 md at the top. Cement formed during two phases of meteoric-phreatic water movement and consists of an early isopachous rim of equant, inclusion free, polyhedral 10-20 μm calcite crystals and a later post-compaction blocky spar with 50-200 μm crystals.
Four pore types are important features of the capping grainstones and packstones. Intergranular pores (1) are primary open spaces in poorly cemented samples. Intercement-rim pores (2) are confined to isopachous rims of microspar. Intra-matrix pores (3) or “micropores” reside between interlocking crystals of micrite within carbonate mud matrix or micrite rims. Finally, intercement spar pores (4) are confined to well cemented grainstones and are minor in importance. Four sizes of pore throats calculated from mercury injection curves access the different pore types: (1) 3-7 μm throats connecting intergranular pores; (2) 1-3 μm throats connecting porosity within isopachous rims; (3) pore throats narrower than 1 μm connecting porosity within lime mud matrix (in micrite rims and also packstones); and (4) interboundary sheet pores/pore throats less than 0.1 μm wide connect porosity between blocky spar crystals.
Significant differences in reservoir character of well-cemented grainstones are likely due to varying proportions of rim- and blocky spar-cement. Permeability associated with blocky spar in coarser grainstones capping each of the sequences is very low, whereas the isopachous rims and minor matrix lower in the sequences are associated with slightly more favorable permeabilities. The drainage efficiency of spar-cemented grainstones is interpreted to be less than that of their rim-cemented counterparts. Finer grainstones with only rim cements have better drainage efficiencies due to wider pore throats which result in lower pore throat/pore space size ratios.
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Carbonate Sands-A Core Workshop
Carbonate sands, both skeletal and non-skeletal, have been studied by geologists as intensely as carbonate buildups. The underlying reason for the studies is the importance of those sands as significant hydrocarbon reservoirs. This core workshop is intended to provide a “hands on” look at the subsurface geologic record of carbonate sands with emphasis on lithofacies, stratigraphy of the sands and surrounding deposits, geometry of the sand deposits, diagenesis and porosity evolution, and wireline log data.