Abstract: Incomplete Carbonate Cementation--Principal Factor in Development of Porosity
Robert G. Loucks
The Lower Cretaceous Pearsall Formation in South Texas has an incomplete diagenetic sequence preserving up to 30% porosity. Initial carbonate sand porosity was reduced by biogenic grain breakage and cementation.
Early diagenesis is similar to that in Holocene and Pleistocene strata. In the marine environment beach-rock cement and micrite envelopes formed. The next stage of diagenesis took place in a series of meteoric phreatic environments produced by partial subaerial exposure. The cements, fine-crystalline equant to bladed rim, medium-crystalline equant, and syntaxial calcite, reflect an oxidizing water chemistry varying in magnesium and low in iron. Leaching of aragonite shells created moldic porosity. Early cements did not affect porosity in the micrite-rich rocks.
Later with initial burial, coarse-crystalline equant calcite cement, low in iron and magnesium, precipitated from a regional meteoric groundwater system. The complete obliteration of porosity in the micrite-rich rocks started during this stage.
Finally at depths over 2,000 ft (610 m) quartz overgrowths, anhydrite, iron-rich baroque dolomite and iron-zoned coarse-crystalline equant calcite were precipitated. Hydrocarbons created a reducing environment. Iron and silicon were released during dewatering of juxtaposed shale which, with stylolitization, produced a hydraulic pump pushing water through the rocks.
Much porosity is preserved in the Pearsall grainstones; however, the subsurface-cementation stage still may be operating. Deeper burial may produce yet another stage of cementation similar to the final void-filling iron-poor calcite cement Davies described as following iron-rich cements similar to those in the Pearsall.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC