Tag Archives: petrography

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

Systematic Petrography Supports Petrofacies Definition and Porosity Distribution Understanding in Pre-Salt Reservoirs

Learn how systematic petrography guided by software helps geologists to define reservoir petrofacies and understand porosity distribution in Pre-Salt Reservoirs.

The Challenge

Understanding the controls and distribution patterns of the quality of complex and heterogeneous lithic pre-salt reservoirs of Sergipe-Alagoas Basin, northeastern Brazil, is of key importance for the optimization of their production.

The Solution

Quantitative petrographic analyses of 135 thin sections performed with the Petroledge® software allowed the definition of reservoir petrofacies according to the main textural and structural attributes, essential primary composition, and main diagenetic processes affecting the types of and distribution of porosity and permeability in the reservoirs.

Background

Pre-salt sandstones and conglomerates of the Sergipe-Alagoas Basin represent rare examples of lithic oil reservoirs rich in ductile low-grade metamorphic rock fragments, such as phyllite, schist, and slate, showing a complex diagenetic evolution. The reservoirs are very heterogeneous, with intercalation of partially cemented porous areas and tight areas intensely cemented by dolomite. The main diagenetic processes affecting the analyzed samples were generated before compaction, in shallow burial conditions, under the influence of ascending thermobaric and alkaline depositional fluids.

Systematic Petrography using Petroledge®

Petroledge® software allows performing detailed petrographic descriptions and interpretations in a systematic workflow, and storing and processing petrographic information within a relational database. An extensive knowledge base works integrated with analytical tools for providing several automatic classification, provenance and diagenesis interpretation.

Results

The systematic petrographic characterization of the pre-salt lithic reservoirs made possible by use of the Petroledge® software, revealed the following:
• Predominance of siliciclastic rocks, medium- to coarse-grained sandstones and conglomerates, massive or with irregular lamination.
• The most abundant detrital constituents are low-grade metamorphic rock fragments (essentially phyllite and schist) and granitic/gneissic plutonic rock fragments.
• Some samples containing ooids, peloids, microbial and recrystallized carbonate intraclasts were classified as hybrid arenites.
• Dolomite is the main diagenetic mineral, mainly filling intergranular pores as blocky and macrocrystalline cement, and replacing grains, locally as discrete blocky crystals.
• Dolomite cementation played an essential role on porosity reduction, where most pores were filled, or preservation, where partial cementation supported the framework, limiting compaction.
• Preserved primary intergranular porosity is much more abundant in the siliciclastic rocks than in the hybrid rocks. Intragranular porosity, mainly from dissolution of feldspars is very abundant in the siliciclastic rocks.
• Mechanical compaction is observed mostly by the deformation of mica grains, metamorphic fragments and mud intraclasts, locally promoting the formation of pseudomatrix.

Systematic Reservoir Characterization and Evaluation

In this study, the influence of diagenesis, depositional texture and primary composition on the quality of the reservoirs was evaluated through the definition of reservoir petrofacies. Dolomite cementation was recognized as the main diagenetic process controlling porosity distribution in the reservoirs. The reservoir petrofacies were separated into four petrofacies associations, according to total porosity, intergranular porosity and cementation: good quality, medium quality, low quality/cemented, and low quality/compacted. Systematic acquisition and processing of petrographic data and information provided by the Petroledge® software supported a better understanding of the distribution of porosity and permeability in the complex lithic pre-salt reservoirs of Sergipe-Alagoas Basin.

Reservoir Petrography - Petroledge
Reservoir Petrography – Petroledge

Author

  • Sabrina Danni Altenhofen – Endeeper

Petrography of the Cenomanian-Turonian transgression in the Potiguar Basin: Petroledge Success Case

This article about petrography and Petroledge was written by Ana Bárbara Sampaio da Costa (Terra & Mar, Geophysics and Geology Solutions). Endeeper gratefully acknowledges Terra & Mar, Geophysics and Geology Solutions.

The Cenomanian-Turonian passage is globally known as the largest marine transgression during the 250 Ma.

In the Potiguar Basin, northeastern Brazil, this passage occurs within the stratigraphic interval constituted by the Açu and Jandaíra formations.

A detailed petrographic analysis of this interval was executed by the description of 190 thin sections using the Petroledge® system for petrography.

The study using petrography was performed through the detailed characterization and quantification of the primary and diagenetic constituents, and pore types.

The reconstruction of the primary detrital composition supported the interpretation of the clastic provenance. The analysis of the diagenetic processes and constituents allowed identifying their impacts on the porosity, establishing paragenetic sequences, and inferring paleoenvironmental information. The identification and quantification of the different pore types made possible to reveal the relationships among the diagenetic processes and the modification of pore space.

The effective integration of petrographic data and information, made possible by use of the Petroledge® system, revealed the following:

  • The predominantly siliciclastic units Açu-3 and Açu-4, from the initial phase of development of the marine transgression, provided from uplifted blocks of the plutonic basement, being affected essentially by diagenetic processes indicative of continental eodiagenesis under dry climate, including mechanical clay infiltration.
  • The late phase of eustatic development (upper Açu-4 unit) is represented by the deposition of hybrid sediments, constituted by extrabasinal grains derived from the uplifted basement, carbonate and non-carbonate intrabasinal grains. Their diagenetic alterations are indicative of eodiagenetic conditions transitional between marine and meteoric.
  • The deposition of the Jandaíra Fm. Characterizes the maximum of the transgression, with establishment of a carbonate platform, which sediments were affected by marine eodiagenetic processes, including intense calcite cementation.

The use of the Petroledge® software was fundamental for assuring the quality and coherence of the obtained data, allowing their effective integration with stratigraphic and sedimentologic information.

Digital Petrography by Petroledge - Illustrative
Digital Petrography by Petroledge – Illustrative

Petrography by Petroledge: Campos Basin success case

This post presents an example of project that uses Petroledge for understanding the Campos Basin rift reservoirs using petrography.

 

Digital Petrography by Petroledge and Stageledge
Digital Petrography by Petroledge and Stageledge

An integrated, seismic-stratigraphic-sedimentological-petrographic project, developed by Brazil’s Rio Grande do Sul Federal University for BG Group, shed new light on the depositional and diagenetic controls on the origin, geometry, distribution, quality and heterogeneities of Campos Basin rift reservoirs and associated lithologies.

The use of the PETROLEDGE® system was vital for the systematic acquisition, storage and processing of petrographic data from the complex and unconventional rocks that constitute the pre-salt, rift section of the basin. “Our understanding of the dominantly intrabasinal nature of the rift sediments and their conspicuous re-deposition by gravitational processes was enhanced by the detailed, yet flexible petrographic descriptions allowed by PETROLEDGE®”, says Dr. Karin Goldberg, head of UFRGS project. Campos rift sediments are essentially constituted by complex mixtures of carbonate bioclasts, siliciclastic and volcaniclastic particles, and stevensite (Mg-smectite) ooids and peloids.

Endeeper PETROLEDGE® system is being used globally by a series of universities and exploration companies, which are taking advantage of the systematic and efficient acquisition and processing of petrographic information generated by the system.