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The Importance of Understanding Diagenesis for the Development of Pre-Salt Lacustrine Carbonates

Abstract

During the past decade, the discovery of the early Cretaceous pre-salt lacustrine carbonates of the south Atlantic have opened up an exciting new opportunity of carbonate reservoirs to examine and explore. Presently, some of the largest carbonate fields in the world exist in these regions and there is still a large yet to find potential. However, being able to move beyond a basic description of the reservoir towards the characterization of a complex depositional system has been difficult and requires multiple analytical techniques to properly describe the reservoir in a three dimensional geologic model. One of these techniques is a full-field core based diagenesis study aimed at describing reservoir quality indicators that could affect how development scenarios are generated as well as preventing excessive up front capital expenditures. In general, the most difficult aspect in describing carbonate reservoirs, from exploration to production, is how diagenesis has altered the rock and ultimately changed the ability for hydrocarbons to be present or how complicated the flow path of hydrocarbons can be. Syn-Rift/Sag lacustrine carbonates in the pre-salt regions generally consist of three main types of reservoirs; 1) Microbialites, 2) Coquinas and 3) Travertines. Depending on the tectonic, climatic and provenance history of a specific region, one could expect to find a combination of one or many of these reservoir types in a geologic section with each demonstrating widely different diagenetic changes. This study presents a detailed core-based diagenetic analysis from three wells in the Campos Basin. The reservoir interval studied was the Aptian lacustrine microbial carbonates. Detailed macroscopic and microscope descriptions were utilized to describe the rock quality and diagenetic pathways of the reservoir. From this, a very detailed paragentic sequence demonstrates numerous episodes of porosity destruction and creation. One of the most interesting results stems from evidence that points toward late stage hydrothermal activity in the region. This has significant implications on understanding the flow characteristics of the reservoir and where best reservoir quality may be located in the field. The results could impact the assessment of reservoir quality for producing fields in the region. Future detailed investigations will be aimed at integrating this data into reservoir models for development activity planning.