Digital Petrography – Fundamental Tool for Understanding Carbonate Reservoirs of Campos Basin

Learn why petrographic characterization is a fundamental tool for understanding Carbonate Reservoirs of the Campos Basin.

The Challenge

Campos basin is the most prolific Brazilian basin. Hydrocarbons are sourced mainly from lacustrine rift section, which also contains important carbonate reservoir rocks. Diagenetic processes strongly influenced the porosity and permeability of these lacustrine carbonates. Understanding the controls and patterns of diagenesis is fundamental for the construction of geologically realistic and effective models for the exploration and production of these reservoirs.

Petroledge Petrography Carbonate

The Solution: Systematic Petrography using Petroledge®

A systematic petrographic study of the rift carbonate reservoirs and associated lithologies was developed in central Campos Basin with use of the Petroledge® software. The petrographic characterization, which comprised all major aspects of depositional structures, textures, primary composition and diagenesis, helped to define the depositional and post-depositional conditions of the succession, as well as the main controls on the reservoirs quality. The Petroledge® system has unique features, designed to facilitate and support petrographic description, as well as automated classifications and multi-format reporting, ensuring efficient and rapid data analysis. Systematic acquisition and processing of petrographic data and information provided by the Petroledge® software allows an optimized use of petrographic information for understanding of the distribution of porosity and permeability.

Geological Context

The origin of the Campos Basin is linked to the initial stage of separation of the African and South American continental blocks in the Early Cretaceous. The initial phase of basin evolution was characterized by rift half-grabens, where fluvial and lacustrine sediments were deposited. The vertical succession analyzed in this study interval is composed of a siliciclastic and volcanoclastic basal section, covered by a complex succession of ooidal stevensite arenites, bioclastic grainstones and rudstones (which includes the reservoirs), and mudrocks.

Results

The integration of the results of the petrography with seismic, stratigraphic and sedimentological information allowed to conclude that:

  • The analyzed rocks are composed of extrabasinal sediments (siliciclastic and volcanoclastic grains and siliciclastic mud) and mainly intrabasinal carbonate and stevensite constituents.
  • The main carbonate rocks correspond to ostracod grainstones and bivalve rudstones, commonly known as “coquinas”, which correspond to the main reservoirs.
  • There is widespread mixing of the bivalve bioclasts with stevensite ooids and peloids. As the precipitation of stevensite occurs only at highly alkaline conditions (pH> 10, high concentration of Mg and Si), which would be intolerable by the bivalves, such mixing would be possible only through re-sedimentation. The distribution of the seismic facies corresponding to the bioclastic deposits and their massive structure indicate that this re-sedimentation took place from different shallow water environments to deep lacustrine settings, though gravitational flows.
  • The mixing of bioclastic and stevensitic constituents has important implications for the quality of the rift reservoirs. Hybrid deposits with significant mixing are commonly strongly cemented, while rudstones with minor or no mixing with stevensitic grains show better preservation of interparticle porosity. These best reservoirs would correspond either to bioclastic deposits in their in situ shallow sites, or to re-sedimented deposits that were not mixed with stevensite sediments.

The systematic petrography of the bioclastic carbonate reservoirs of Campos Basin allowed by the Petroledge® software was essential for the understanding of depositional and post-depositional conditions of the rift succession, as well as of the main controls on the quality of the reservoirs.

Author

  • Sabrina Danni Altenhofen – Endeeper