Abstract: Carbonate Reservoir Rocks from The Bahamas Continental Margin

Valladares Amaro, Silvia; Dania Brey Del Rey; José Alvarez Castro - Ceinpet, Cuba

Carbonate reservoirs have porosity formed in low to moderate energy environment with episodic high-energy environment close to platform reaching turbulent stage in slope areas. Porosity is intramicritic occluded by mudstone with peloids or mosaic of idotopic and xenotopic dolomite in shallow water (Jurassic ? Lower Cretaceous) or occluded by micrite in deep waters. Reef derived rocks have biomicritic and intrabiomicritic porosity. This porosity was not occluded but was affected by recrystallization.

Some diagenetic processes worsened porosity. The main processes are calcite deposition in basinal and slope rocks and dolomitization in platforms. Less important processes are related to deposition of gypsum, anhydrite, barite, pyrite and spatic calcite. Compaction affected porosity twice: once during the deposition in the original basin and the second during tectonic stacking by thrusting. At 2700 m a break of porosity is observed in all sequences.

The diagenetic processes that enhance porosity are mainly fracturing and recrystallization. Bellow porosity break fracturing creates reservoir, which are related to productive horizons. Paleokarst and leaching, since the eogenesis, also affected a large volume of rocks. Idiotopic dolomitization improves intercrystalline porosity.

The classification of reservoirs considers sedimentation environment and diagenetic processes. We divided carbonate section in three periods. The reservoir characteristics in each period are:

Oxfordian - Kimmerigdian. The paleobasin had no more than 100 m water depth. Because of compaction and low fracturing a part of this section have no reservoir horizons. On the other hand, Lower Kimmergdian carbonates have several reservoir horizons thanks to intense fracturing and karst. This is a productive section in Boca de Jaruco oil field.

Tithonian - Neocomian. The basin at this moment had two main environments carbonate platform and deep-water basin. Carbonate platform sediments presents porosity in dolomite and carbonates. In some levels porosity was destroyed by intense dolomitization. Different types of rocks represent slope sediments, which have variable porosity. In batial sediments reservoirs are due to the fracturing and karst with high values of porosity and permeability. This is the productive section in Varadero, Boca de Jaruco, and Puerto Escondido oil fields, as well as the new discovery Cupey.

Aptian - Maestrichtian. The paleobasin at this time is similar to the previous except the breakdown of the platform. The sediments in the channels are mainly batial represented by deepwater carbonates and marls. This section is seal prone because of the low level of fracturing. Platform sediments are less dolomitized emerging reefal bodies with high porosity. Slope sediments improve reservoir characteristic toward Upper Cretaceous. Basinal carbonate section of this period still has several reservoir levels due to fracturing and leaching. This section produces in several oil fields.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil