Diagenetic Model Related to Unconformities Surfaces and Application in the Exploration of Deep Reservoirs in Sergipe-Alagoas Basin, Brazil

Sergipe-Alagoas basin, in northeastern region of Brazil, is one of the oldest petroleum basins in operation in this country, with its first exploratory activities in the 30. The geological complexity of the basin is related to exploratory difficulties but also a series of attractive possibilities. Studies were conducted to understand the geological context related to presence of significant porosity values at great depths, like 4200 m deep, where values of 20% of porosity was find, in offshore shallow water. An important contribution for these studies was developed on the Serraria Formation reservoirs, resulting in predictive diagenetic models to hydrocarbon exploration. These studies integrated lithofaciologics analysis, depositional conditions, paleogeographic and paleoclimatic reconstructions and the burial history control on the diagenetic evolution and porosity distribution. This way permited recognize important factors responsible for best porosities in Serraria reservoirs in some oil fields. Two deep reservoir models of increment and/or preservation of porosity in deep reservoirs conditions can be recognized. The conditions of each model can occur combined with consequent improvement of its efficiency: The Caioba Model was directly related to direct meteoric fluids infiltration in sandstone reservoirs during an exposition time related to uplift phase in part of the basin. The solvent action of the meteoric water infiltrated from areas of unconformities surface and/or along fault zones, provided considerable increase of porosity by dissolution of feldspar grains and carbonate cements. The Furado Model, was related to the early occupation of reservoir by oil. The process of preservation of porosity by early occupation by oil can be extremely efficient, since the closing of the structure, persist throughout later burial history. In terms of Petroleum System Analysis two sets of reservoirs are considered: 1) Pre-salt Reservoirs and 2) Post-salt Reservoirs. To Pre-salt Reservoirs oils from continental and marine source rocks are presents in different sectors of the basin under different burial history controls. Significant oil accumulations could be preserved in basin sectors where the seals superimposed (salt, for example) were preserved. The Post-salt Reservoirs, are positioned relatively distant from the main source rocks intervals, situated below the salt. Only accessible by migration routes associated with the faulted and basculated blocks.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California