Click to view complete article.

Diagenetic Study Applied to Maruim Member (Riachuelo Formation), in the Onshore Part of the Sergipe Sub-Basin, Brazil

Mary Luz Raigosa Diaz¹, Jose Ramon Mas Mayoral², and Egberto Pereira³
¹Baker Hughes, Rio de Janeiro, Brazil
²Universidad Complutense de Madrid, Madrid, Spain
³Universidade do Estado do Rio de Janeiro, Brazil

The main objective of the study was the reconstruction of the diagenetic evolution of the carbonates within the upper portion of the Albian, Maruim Member in the Sergipe Sub-basin (northeastern, Brazil). Special emphasis was placed on the dolomitization, because the dolomites exhibit the best reservoir rock properties. Correlation of the outcrops and the integration of the petrographic, cathodoluminescence, SEM and geochemical (elemental and isotopic study) analyses allowed the reconstruction of the diagenetic history of the studied interval. Carbonates of the Maruim Member are affected by diagenetic processes of the eogenetic, mesogenetic and telogenetic phases.

The upper portion of the Maruim Member (Riachuelo Formation), consist of shallowing upward depositional cycles. From outcrops study and petrographic analysis, the depositional environment was defined as a carbonate platform with a lagoon area and a high energy region composed of carbonate banks. The lagoon area consists of packstone/grainstone with pellets, intraclasts and bioclasts. The high energy banks consist of oolitic grainstone and grainstone with ooids and oncoids. The carbonate bank facies are characterized by the low content and variety of bioclasts. Dolomitization was one of the main diagenetic events of the eogenetic phase and dolomite replaces all or part of the limestone of the Maruim Member. The dolomitization is concentrated at the top of the depositional cycles and gradually decreases toward the base. The relationship between porous space and dolomitization was studied based on observations of the crystalline dolomite fabric from petrographic observations. A common feature of the dolomites is a planar fabric (euhedral to subhedral crystals) which is favored by low temperature conditions.

The key conclusions are:

1. The main microfacies observed in the studied interval were oolitic grainstone and oncolitic/oolitic grainstone deposited in high energy banks. The microfacies of low energy were deposited in lagoon area and are composed of packstone/grainstone with pellets, intraclast and bioclasts.
2. The top of the cycles of the carbonate banks of high energy with total dolomitization, exhibit the best reservoir properties.
3. The dolomites underwent two stages of porosity development; the first stage corresponds to intercrystalline porosity, developed during the dolomitization process (eogenesis phase). The second stage includes secondary porosity developed during the dissolution in the telogenesis phase (vuggy porosity creating hollow dolomites).
4. Overall the porosity of the carbonate sequence of the Maruim Member is greatly reduced by cementation and mechanical and chemical compaction.

AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012