Exploration of complex deposits in frontier and deep prospects. As exploration advances to new, deeper, depositional or tectonically complex frontiers, simple and conventional deposits are becoming scarcer. Despite the spectacular advances in seismic and log acquisition and interpretation, the risks involved in exploring new, deeper, depositional, tectonic or diagenetically complex prospects remain great. 3D and 4D models incorporating sophisticated structural, thermal, generation and migration histories help reduce exploration risks in new and tectonically complex environments, but simply lack information about an essential part of petroleum systems: the reservoir. Most of the new prospects represent clastic, carbonate or other unconventional reservoirs with strong diagenesis. For these, the conventional indirect evidence of texture and composition provided by logs and conventional simplified petrographic descriptions is simply not enough. Undiscovered and unproven hydrocarbon reserves are contained in conventional carbonate and clastic reservoirs with strong diagenesis, such as the deep Gulf of Mexico and subsalt prospects in Brazil, or in unconventional formations, such as tight sands, gas shale, and methane from coal layers. In fact, about 35% of technically recoverable reserves in the US are in unconventional fields. There is an emerging trend for the integration of systematic petrographic analysis with petrological, geochemical, wireline, petrophysical and seismic analysis, in order to address the challenges involved in reducing exploration risks and optimizing development and production from these deposits.
Increased recovery of old fields
A critical front in the struggle to meet rising energy demand involves finding ways to extract more oil from old fields with declining output. Breakthrough performance involving new concepts, new data, and new technologies is needed to reverse or slow downward trends in production. This involves gaining a better understanding of these reservoirs to redesign or adjust secondary or tertiary recovery programs or, in some cases, to extend, drill or recomplete new, unconventional and/or previously neglected reservoirs. Both goals require advanced and detailed reservoir characterization using high-resolution stratigraphy and systematic petrography. Enhanced recovery projects in mature oil fields require detailed petrographic characterization to understand heterogeneities and controls on pore geometry and permeability, such as in the Lower Tertiary reservoirs of the Talara Basin, Peru. Detailed reservoir heterogeneity and quality models that incorporate detailed petrographic data are essential for designing engineering solutions to optimize reservoir development.
The problem: understanding complex and unconventional reservoirs
Deep, unconventional and complex deposits are becoming the main exploration targets. Some hints of the new rules of the exploration game come from offshore areas in the Gulf of Mexico and on the eastern margin of Brazil. In the Upper Tertiary section of the Gulf, seismic amplitude anomalies very similar to oil-saturated sands have been unsuccessfully drilled since the late 1970s. These anomalies were recently found to correspond to sands containing a substantial amount of ash. volcanic, arising from large-scale eruptions in the Yellowstone area through the Mississippi system (Totten et al., 2005). The same type of false DHI (direct hydrocarbon indicator) led to dry wells in the marginal basins of eastern Brazil, where anomalies were related to altered and reworked glassy volcanic fragments produced by submarine eruptions (hyaloclasts). Significant amounts of money could have been saved in these cases if adequate petrographic characterization had been performed to calibrate wireline and seismic logs immediately after drilling the first dry hole. In the Santos Basin, offshore Brazil, Late Cretaceous reservoirs show abnormally high porosity preserved at great depths by the inhibitory effect of chlorite rims on quartz cementation and compaction (Anjos et al., 2003;) . The same effect was also identified in Jurassic reservoirs in the Gulf area (Thomson, 1982; Dixon et al., 1989) and in the North Sea (Ehrenberg, 1993). In these cases, the importance of systematic petrographic surveys and petrological analyzes lies in modeling the spatial distribution of chlorite (hence the related porosity conservation) in relation to parent materials, such as volcanic and other grains. Fe-Mg, and the early diagenetic clays. such as smectite or odinite. Recent discoveries of gigantic early Cretaceous subsalt accumulations of oil and gas in the deep and ultra-deep areas of Brazil’s eastern marginal basins have brought petrographic studies to the center of the arena. Subsalt reservoirs are very complex and intensely affected by diagenetic processes, and their genetic conditions and relationships with facies and primary compositional controls are not yet known. Previously drilled subsalt carbonate and clastic reservoirs in the shallow water portion of the Campos Basin are characterized by extreme variation in porosity, as well as diagenetic processes and products. Systematic studies should be developed that consider evidence and petrological, geochemical and stratigraphic models to evaluate the risks through the prediction of the quality of the reservoir. In these studies, advanced petrographic characterization will be of vital importance.
The solution: effective and systematic petrographic surveys
As the porosity and permeability of most complex, unconventional and mature reservoirs (as well as their seismic and log signatures) are strongly controlled by diagenesis, the importance of incorporating petrographic characterization into the exploration and production culture becomes clear. becomes obvious. Within this new scenario, the systematic acquisition and use of petrographic data to calibrate the indirect characterization and reservoir quality assessment provided by seismic and logging allows attributing the real meaning of the rock to the facies of the logs already seismic units and reflectors. Reservoir quality studies in complex reservoirs have shown that habits, location, spatial and temporal distribution of diagenetic phases are more important than constituent types. Therefore, there is an emerging trend to integrate microscale petrographic analysis with log analysis, petrophysics, and seismics. The integration of petrography into reservoir quality models is a crucial advance that will guide exploration and production going forward.
There are two main limitations to the effective integration of petrographic information in E&P: (i) most petrographic data is scattered in spreadsheets and/or text reports, with variable formatting and often imprecise organization and terminology, and ( ii) software packages commonly used for well log analysis, seismic interpretation and 3D modeling do not import petrographic data in these formats. The need to organize detailed petrographic data for effective and systematic use in E&P has led to the development of the Petroledge® system (De Ros et al.,
2007). Petroledge®, an intelligent database system that combines relational database resources and knowledge-based expert systems, was created to support the systematic analysis, storage and processing of detailed petrographic information on primary structures, textures and constituents, and mainly on habits, location and paragenetics. ratios of diagenetic constituents and types of pores. Such systematically organized petrographic information can be effectively integrated into 3D reservoir models and flow simulation software.
reservoir petrofacies
The intrinsic quality of oil reservoirs (porosity, permeability) is controlled by key petrographic parameters, namely depositional structures, textures and composition, diagenetic processes and products (volume or intensity, habits and distribution), as well as the types and pore distribution (De Ros & Goldberg, 2007). The reservoir petrofacies concept is the most effective way to integrate and incorporate such key petrographic information into reservoir quality characterization and prediction. The petrofacies of the reservoir correspond to ranges of defined values of
porosity and permeability, as well as characteristic logs and seismic signatures. Consequently, they can be used to calibrate logs and seismic with real rock properties. Reservoir petrofacies-calibrated logs can be applied to realistic representation in 2D sections and 3D models of reservoir quality and heterogeneity, leading to improved static and flow simulations during development and production. Seismic facies and reflectors calibrated by reservoir petrofacies will improve the accuracy of seismic interpretation in deep, new or unconventional settings.
challenges
Building sensitive 3D and 4D models incorporating RQ prediction through advanced petrographic characterization is indeed a new frontier in exploration. Such developments will enable much more sophisticated risk assessment during the exploration of complex and unconventional deposits in frontier and deep areas. Another challenge involving the use of advanced petrographic characterization is the integration of geological knowledge. The effective extension and integration of data generated with different tools (and stored in different formats), such as seismic, logs, petrophysics and petrography, will involve much more than the mere integration of files and formats. A much more complex problem will be the semantic integration required at the application level. The different tools not only have a different representation of the geological objects to be processed, but also capture different points of view and even meanings of these objects. Integration at the insight level is required to provide a consistent view of data to support exploration decisions. However, with the growing awareness of the need for a petrographic database that can provide the various software packages commonly used in exploration and production, the gap between petrography and modeling will narrow. The appropriation of systematic petrographic characterization in realistic reservoir quality models will allow a better assessment of exploration risks and optimized production procedures. The challenges involved in exploring and producing complex reservoirs will be overcome with the intelligent integration of petrography into the E&P operational workflow.