Tag Archives: petroleum reservoir

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 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.