--> Abstract: Diagenesis and its Influence on the Creation or Destruction of the Porosity in Carbonate Sequences from Placetas TSU, by D. Brey del Rey and J. Hernandez-León; #90933 (1998).

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Abstract: Diagenesis and its Influence on the Creation or Destruction of the Porosity in Carbonate Sequences from Placetas TSU

Brey del Rey, Dania and J. Hernandez-León - Ceinpet, Cuba

This work analyzes the diagenetic processes that caused the porosity creation or destruction in carbonate sequences, ranging from Upper Jurassic to Upper Cretaceous in Placetas TSU. For this purpose the pore space in cores and in thin sections were studied.

In the post-depositional stage the sediments suffered different diagenetic processes: recrystallization, compaction, dissolution, cementation and fracturation; which in a sense deteriorate or improve porosity, considering microfacies factors. According to Choquette & Pray (1970) scheme, it was established that these carbonate formations obliterated their primary porosity space healed by peloidal mudstones and those deposited in deep waters do it by micrite which acts as a syn-sedimentary cement. This determined the ultimate constitution of rocks. During diagenesis the porosity was favored by dissolution (vug and channels) and recrystallization (intercrystalline. Parallel to these, porosity was reduced or totally disappeared because of calcite or some dolomite cementation.

The rocks began to be buried during mesogenesis. Compaction and cementation occurred here, which are negative processes for porosity development. Recrystallization continues and the pressure solution stylolites appear. These processes culminated in telogenesis and the rocks illustrate the actual porosity. Then began tectonic activity, that uplifted the rocks until they were exposed to meteoric and phreatic waters. This resulted in the vadose zone, dissolution taking place and development of the karst structures. The fracturaction was strong, producing brecciation and microbrecciation that gave the rocks fracture and interparticule porosities. The dissolution continued opening the fractures, originating vugs. The spathic calcite cementation in fracture and vugs, together with recrystallization, obliterated the intercrystalline porosity; and the bioclasts dissolution resulted in the moldic porosity type.

A permanent characteristic in all carbonate formations is the presence of three fracture generations, related to the three stages of diagenesis:

First generation: Dispersed fractures, ramified in chaotic nets healed with spathic calcite; someone of them could be open again and newly cemented or partially filled with bitumen. These fractures were formed during eogenesis or mesogenesis.

Second generation: Parallel fractures to bedding planes which can be partially empty or filled with bitumen. Polished or striated silken sides are observed with bitumen or intense oil odor. In zones they occur producing microbrecciation. These fractures are the main ones because they offer the greatest permeability in reservoirs.

Third generation: The fractures cut normally the latters. Megascopically are best seen, often calcite cemented and microbrecciated locally. These fractures offer vertical communication to reservoirs.

All generations belong to different degrees of telogenesis.

Conclusions:

The present microfacies belonging to shallow and deep waters have the main porosity as the fracture type, besides being improved by the intercrystalline, moldic and interparticule types. This latter occurs in brecciated zones of intense fracturation.

Different porosity types can be related to the K vs Ø graphic and it produced qualitative reservoir values.

Different obliterations and improvements of porosities were established according to different diagenesis stages, mesogenesis and telogenesis being the main ones.

All carbonate formations have three fracture generations, the second and third ones allow communication to reservoirs.

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