--> ABSTRACT: Lithification and Mesogenetic Porosity Evolution in Lower Permian Periplatform Limestones, Midland Basin, Texas, by S. J. Mazzullo; #91020 (1995).

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Lithification and Mesogenetic Porosity Evolution in Lower Permian Periplatform Limestones, Midland Basin, Texas

S. J. Mazzullo

Lower Permian (Lower Leonardian) periplatform strata in the eastern Midland Basin include resedimented megabreccias and turbidites which are prolific hydrocarbon reservoirs in stratigraphic traps here and elsewhere in the Permian Basin. Prior to redeposition, platform strata were exposed subaerially, the result of which initially included porosity formation via meteoric dissolution. Porosity ultimately was occluded, however, by calcite cements of depleted ^dgr18O composition (-2.6 to -6.6^pmil), and with vadose and dominantly phreatic meteoric habits.

Redeposition transported both megabreccias, composed of blocks of lithified and stabilized limestone, and unconsolidated, aragonite-poor sediments (turbidites), into slope and proximal basin locations. After some mechanical compaction but prior to significant chemical compaction in the turbidites, aragonitic grains were dissolved concurrent with pervasive calcite cementation. These diagenetic processes likely occurred in the relatively shallow-burial, sub-seafloor environment. Cements occur as overgrowths on crinoid fragments, and in interparticle and biomoldic pores, and are believed to have originally been high-Mg (about 12 mole% MgCO3) rather than low-Mg calcite. Aragonite dissolution by cold, undersaturated marine pore fluids is considered unlikely because estimated max mum water depths (460 m) were well above the lysocline. A mineral-controlled model of aragonite dissolution-calcite precipitation also is considered unlikely because of aragonite deficiency in the sediments. Instead, the precipitation of calcite cements from circulating marine fluids may itself have lowered carbonate saturation states and promoted aragonite dissolution in much the same manner as is suggested for some Cenozoic, shallow-buried carbonates above the lysocline. The mean ^dgr18O composition of the Permian cements (-3.1^pmil) is depleted rather than enriched relative to extant sea water, however, and likely reflects later isotopic re-equilibration accompanying Mg loss, which occurred during the Permian at an approximate burial depth of 610 m.

One of the last diagenetic events affecting the rocks, after they were buried to at least 2500 m depths, was extensive dissolution porosity formation. The most likely source of undersaturated fluids capable of dissolution was acid fluids generated during hydrocarbon maturation in associated shales. Mean vitrinite reflectance value of these shales (0.70%) indicates that they are mature, and burial depth-temperature reconstructions suggest that maturation, and presumably porosity formation, began during the Triassic.

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