Facies specificity of megaporosity in Mesozoic shelf-edge carbonates

F.O. Meyer

Shell Development Company, Houston, Texas

Cores from three wells drilled by Shell Offshore Inc. provide insight into the evolution and distribution of porosity within the Mesozoic shelf -edge carbonates of Baltimore Canyon basin. Significant primary and secondary megaporosity was found only in the late Kimmeridgian-Berriasian aggradational part of the paleoshelf-edge deposits. Megaporosity is patchy but shows an overall seaward decrease in abundance, averaging from about 13 % in the back-reef rocks to about 4 % in reef-front deposits. This porosity trend parallels a basinward change from mainly freshwater to predominantly submarine diagenesis. In grain-supported reef-flat rocks, the porosity consists of some primary pores but dominantly is of secondary moldic pores. The secondary pores are fabric selective after former aragonitic fossils. Both primary and secondary pores are partly or completely filled with freshwater cements. These consist of clear, equant calcite crystals distributed as continuous circumgranular crusts and epitaxial rims typical of stabilized grainstones. Cementation was contemporaneous with or postdated leaching, but predated breakage of empty micritic envelopes. In reef-front deposits, megaporosity occurs as delicately dissolved fossils and porosity haloes in cemented packstones and grainstones within constructional reef cavities. These secondary pores are surrounded by at least two generations of cement. The first generation is a synsedimentary marine phase. It consists of both clear-bladed and inclusion-rich radiaxial crystals distributed chiefly as pore-lining crusts. The second generation is nonmarine, similar to the freshwater cements found in the reef-flat facies.

Meyer, F. O., 1986, Facies specificity of megaporosity in Mesozoic shelf-edge carbonates from the Baltimore Canyon Basin: American Association of Petroleum Geologists Bulletin, v. 70, p. 621.