Brian W. Wilhite1,
(1) Wichita St. Univ, Wichita, KS
(2) Wichita St. Univ
Abstract: Foraminiferal sand shoals in Chetumal Bay, Belize: Modern analog of some Paleozoic hydrocarbon reservoirs deposited in wind-dominated, microtidal, sheltered environments
Holocene foraminiferal sands and sandy muds, deposited unconformably on a surface of karsted Pleistocene limestone and, locally, on buried soil, are present in Chetumal Bay ( < 3 m deep) to the lee of Ambergris Caye in northern Belize. The sediments cover 300 km2, and catch-up and keep-up, shallow-water deposition began ~6000 yrs BP with rapid transgression of the northern Belize shelf. Sands and gravelly sands are deposited mostly in shallow water over bedrock highs and/or mud shoals, and define sediment bodies of linear and sheet-like geometry. Surface bedforms include dune sets locally with stacked ripple-forms, overwash lobes, and spits resulting from sediment transportation to the S and W by currents generated in response to storms ("northers") and easterly trade winds, respectively, in this microtidal ( < 0.3 m) environment; similar bedforms are inferred in the subsurface. In contrast, sandy muds are restricted to adjoining, relatively deeper water areas overlying bedrock lows, and are heavily bioturbated by infauna and marine grasses. Primary porosity includes interparticle and intraparticle pores in the sands, and mainly intraparticle pores in the sandy muds; effective porosity and permeability increase upward in the section. As much as 65% dolomite is present locally in the sandy muds, however, and it has precipitated as a result of anaerobic bacterial sulfate reduction and methanogenesis. Dolomitization ultimately may act to enhance matrix porosity in these deposits. Facies relationships, local progradation of sands over muds in overall response to decrease in rate of sealevel rise, and syndepositional dolomitization give rise to complex yet somewhat predictable heterogeneity in reservoir-analogous lithologic and porosity units.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana