Multidisciplinary Characterization of a Polyphasic Albian Karst, Campos Basin, Brazil presented by Joao Menescal

Abstract

Carbonate reservoirs frequently show karst features that substantially impact reservoir fluid flow and recovery factor. It is essential to define the karst conceptual model and its spatial distribution in order to obtain a reservoir model that allows a good production forecast and reservoir management. This work presents the karst characterization process based on the description of resistivity and acoustic image logs, well cores, petrographic sections, seismic attributes and production data from the Tartaruga Verde Field, Campos Basin, and the workflow for karst representation in a 3D geological model. The Albian reservoirs from the Campos Basin are mostly composed of oolitic- and oncolitic grainstones and packstones accumulated in a shallow marine platform, immediately overlying a thick Aptian, evaporite section. The lower part (up to 2/3) of the Albian reservoir section is completely dolomitized. The karst features are preferentially developed in the dolomitized section. They were generated by meteoric water infiltration during exposure periods at the highstand system tract. They are associated to fractures generated by salt tectonics triggered by the carbonate platform development over the plastic salt layer. The formation rheology seems to be an important controlling factor of this diagenetic imprint since the alternation of dolomitic intervals with hybrid carbonates control the intensity of fracturing and consequently the intensity of water infiltration and degree of dissolution. There are also important hypogenic karst features close to the top of the Albian reservoir, where the dolomitization is reduced. These dissolution features crosscut stylolites indicating that the ascension of hydrothermal fluids along reactivated rift faults occurred during mesodiagenesis. They occur preferentially along the faulted borders of the carbonate platform, sometimes resulting in seismically identified, extensive collapse features. The process of karst characterization is crucial to support the next step of karst reservoir modeling, which is currently being done for both pre- and post-salt carbonate reservoirs by using a workflow defined in a recent R&D project at Petrobras. The representation of these dissolution features as both a set of DFN enlarged fractures, and as a second step of facies simulation superimposed to the original matrix grid is essential to better represent the rock petrophysical properties and to obtain a more reliable production forecast.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018