Facing the Challenge

Author Luiz Fernando De Ros, Karin Goldberg, and Mara Abel

Complex and unconventional reservoirs provide important reserves to the world's energy supply however, they are often challenging to produce. Systematic petrographic studies take a close look at these reservoirs yielding improved characterization and quality prediction; a crucial breakthrough that will guide future exploration and production

The recent discoveries of giant, early Cretaceous, sub-salt oil and gas accumulations in the deep and ultra-deep areas of the eastern marginal basins in Brazil have helped to bring petrographic studies to the center arena. Sub-salt carbonate reservoirs previously drilled in the shallow-water portion of the Campos basin, shown here, are characterized by extreme variation in porosity and in diagenetic processes and products. Recrystallized mollusk bioclasts provide the heterogeneous framework for these lacustrine reservoir rocks. ©Endeeper

There are two types of new exploration targets: Conventional clastic and carbonate reservoirs with strong diagenesis, located in deeper, depositional or tectonically-complex new frontiers or unconventional plays, such as tight sands, gas-shales and coalbed methane. In spite of the spectacular advances in the acquisition, processing and interpretation of seismic and log data, the risks involved in the exploration of these new prospects using these conventional tools are huge. 3-D and 4-D models incorporating sophisticated structural, thermal, generation, and migration histories have helped to reduce exploration risks in new and unconventional settings however, they simply lack information on an essential part of petroleum systems: the reservoir.  

The conventional approach for dealing with the reservoir rocks through the indirect evidence of texture and composition provided by the logs, with occasional help from simplified petrographic descriptions of core or sidewall samples, is simply not adequate for complex and unconventional reservoirs.

The first example illustrates the importance of calibrating logs and seismic with rock data. In the Gulf of Mexico upper Tertiary section, seismic amplitude anomalies very similar to oil-saturated sands that have been unsuccessfully drilled since the late 70's. Only lately, it was discovered that these anomalies correspond to sands containing substantial amounts of volcanic ash derived from large-scale eruptions in the Yellowstone area through the Mississippi River system. The same type of false direct hydrocarbon indicator led to dry wells in the eastern Brazilian marginal basins, where the anomalies were related to reworked, altered vitreous volcanic fragments (VF) produced by submarine eruptions (hyaloclasts). Large amounts of money could have been saved in both cases had a proper petrographic characterization been performed immediately after the first dry well was drilled. © Endeeper

The second example shows the importance of petrographic studies for the development of reservoir quality predictive models. Upper Cretaceous reservoirs from the Santos basin, offshore Brazil, as well as Jurassic reservoirs from the Gulf of Mexico and the North Sea contain anomalously high porosity preserved at great depths (>4,000 m). Petrographic studies revealed the inhibiting effect of chlorite rims (green) on quartz cementation and compaction. Systematic petrographic studies allow modeling of the spatial distribution of chlorite and hence the related porosity preservation. © Endeeper

The third example shows how petrography can help companies squeeze more oil out of old fields with declining production. A better understanding of the "old" reservoirs can help to redesign or adjust the secondary or tertiary recovery programs; or in some cases, to extend, drill for, or re-complete new, unconventional, and/or previously neglected reservoirs. Systematic petrographic characterization has helped to plan the enhanced recovery program for mature lower Tertiary reservoirs of the Talara Basin, Peru. Detailed petrographic characterization allowed researchers to understand the heterogeneities and controls on pore geometry and permeability, and was essential for the design of engineering solutions for optimizing reservoir exploitation. The photo shows multiple smectite (swelling clay) rims (green) generating very complex pore systems (blue). © Endeeper

Using effective petrography to solve complex reservoirs

The increasingly sophisticated petrologic studies of reservoirs and associated rocks incorporate, among other techniques, analyses of major and trace chemical elements through electron and ionic microprobes, stable and radiogenic isotopes, fluid inclusions and mineralogical analyses through X-ray diffraction and electron microscopy. However, the affectivity of these advanced methods depends on systematic petrographic studies through optical microscopy of the depositional structures, textures and composition, and of the diagenetic products and evolution of these rocks. The importance of petrographic studies for the characterization and modeling of complex reservoirs can be appreciated from three different cases illustrated in the following photos.  

Empowering Knowledge

The deep sub-salt reservoirs off the Brazil coast are very complex and intensely affected by diagenetic processes after deposition, which genetic conditions and relationships with facies and primary compositional controls are yet to be understood. Systematic reservoir quality prediction studies considering petrologic, geochemical and stratigraphic aspects and the integration of advanced petrographic characterization with log analysis, petrophysics, and seismics will be of key importance for the assessment of risks during the exploration of these and other complex reservoirs.

Petroledge®, a new generation of software, was developed by the researchers at Endeeper to perform detailed petrographic descriptions and interpretations in a systematic workflow, and to store and process the petrographic information within a relational database. This software allows the easy acquisition and retrieval of petrographic information and its integration with other applications, such as 3-D reservoir modeling and flow simulation software.

We are entering many new frontiers in exploration and production. The incorporation of petrographic characterization into robust, realistic reservoir quality models is one more tool that will result in substantial increase in the exploration success and production recovery from complex reservoirs.

The Authors

The authors are experts in Petrology and knowledge systems. Their work with Petrobras and other companies have led to the development of an intelligent database system that allows the integration of petrographic studies into reservoir models, as explained in the last section of this article.

Karin Goldberg is a researcher of Sedimentary Petrology in the Petroleum Geology Program of the Institute of Geosciences at the Federal University of Rio Grande do Sul. Luiz Fernando De Ros is professor of Sedimentary Petrology at the Institute of Geosciences, UFRGS and a researcher at Endeeper Reservoir Knowledge Systems. Mara Abel is a professor in Knowledge Engineering at the Institute of Informatics of UFRGS and Technical Manager of Endeeper.

UFRGS The Institute of Geosciences of Rio Grande do Sul Federal University (UFRGS) has a long tradition in petroleum geology research in cooperation with Petrobras and other companies. UFRGS Institute of Informatics runs the largest graduate Computer Science program in Brazil. Endeeper is a company created by researchers from the Geosciences and Informatics Institutes that develops innovative software systems for dealing with strategic information in petroleum exploration and other knowledge-intensive domains.