Dolomitization as Control on Reservoir Quality--Case Study: Block III, Lake Maracaibo, Venezuela

Xiomara Marquez

Paleocene carbonate sequences in Block III, Lake Maracaibo, have always been neglected, even though similar sequences such as those reported in the Smackover Formation, Gulf of Mexico coastal plain, USA, have shown great hydrocarbon potential.

Core study in well VLC-950 shows that the Paleocene carbonate sequence consists of four oolitic facies: skeletal-oolitic rudstones (C1), oolitic packstones/grainstones (C2), oolitic grainstones (C3), and sandy oolitic grainstones (C4), all deposited in a prospective shallowing-upward carbonate sand sequence.

Diagenetic processes such as micritization, calcite cementation, solution, dolomitization, and compaction have modified the original textures of these facies. It seems that dolomitization has been the most important in controlling effective porosity. Dolomitization has occurred in the whole sequence. In C1, it is represented by isolated, fine, euhedral crystals. In C2, dolomitization has selectively replaced the oolites with a mosaic of coarse, anhedral crystals. In C3, dolomitization has selectively replaced the calcite cement and to a much lesser extent the oolites. Oolites not dolomitized have been selectively leached, resulting in well-developed oomoldic porosity that forms the reservoirs. In C4, dolomitization is minor, and it is represented by isolated, very fine crystals.

Petrophysical analysis revealed that in C3 the porosity value is 19%, permeability is 1.21 md, and oil saturation is 80%; in C1, C2, and C4, porosity averages 4.3%, permeability is 0.06 md, and oil saturation is negligible.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.