ABSTRACT

Porosity at North Haynesville 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 a shelf slope where these sediments were largely confined to local shoals resulting from contemporaneous uplift. A calcarenite bar along the seaward edge of the slope restricted circulation, and North Haynesville was situated behind this barrier.

Because of hypersalinity, Smackover allochems are almost entirely nonskeletal, and deposition resembled that of the Bahama Banks. Pellet-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 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 the porosity even further.