Environmental Control of Porosity in Upper Smackover Limestone, North Haynesville Field, Claiborne Parish, Louisiana: ABSTRACT

Abstract

Present porosity of the reservoir rock in North Haynesville field is limited to primary intergranular pore space. Certain facies contained larger, better connected pores as a result of deposition in a higher energy environment, but preservation of this porosity against cementation and pressure solution makes the diagenetic environment equally critical. Regionally, Smackover porosity is confined to a coastal shelf where calcarenites with primary intergranular porosity were widespread, and to a shelf slope where the calcarenites were confined largely to local shoals resulting from contemporaneous uplift. A calcarenite bar along the seaward edge of the shelf slope restricted circulation. North Haynesville field is behind this barrier. Because of hypersalinity, Smackover allochems are almost entirely nonskeletal, and deposition resembled that of the Bahama Banks. Pelletal mud was deposited in quiet, shallow water with probable periods of subaerial exposure. Slight differential structural movement permitted turbulence in local areas where a mixed facies was deposited. Further movement, probably including lowering of the seaward barrier, created conditions favorable for extensive oolitic accretion during deposition of the reservoir facies. This turbulent shoal environment produced clean, well-sorted calcarenite with excellent porosity, much of which was destroyed through cementation with sparry calcite, possibly under subaerial conditions. A completely cemented zone at the top of the reservoir facies prevented further entry End_Page 2032------------------------------ of dissolving or precipitating waters. Evidence of compaction is common in the reservoir, but early partial cementation and migration of oil prevented excessive compaction by retarding pressure solution. The lower energy mixed-facies environment produced an unsorted calcarenite with abundant mud matrix and poor porosity; pressure solution reduced porosity even further. End_of_Article - Last_Page 2033------------