Petroleum Geoscience - Volume 18, Issue 2, 2012

Gravity data were analysed in the Ghadames Basin and surrounding regions in southern Tunisia in order to determine the basement structure of the region and its relationship to petroleum exploration in relatively unexplored basins. The analysis included the construction of regional Bouguer gravity anomaly and horizontal gravity gradient maps. These maps indicate that the Ghadames Basin is not a simple sag basin but consists of a series of sub-basins and uplifts. The northern boundary of the basin which we call the Telemzan–Ghadames transition zone is marked by a NE-trending high amplitude gravity gradient anomaly which decreases in amplitude toward the east and breaks into a series of north–south- and east–west-trending anomalies implying a more structurally complex region. When the known petroleum fields are overlain on to the gravity gradient anomaly maps, the fields mostly occur along or next to linear alignments of horizontal gravity gradient maxima. We interpret the correlation of the petroleum fields and horizontal gravity gradient maxima to indicate that the basement was involved in forming the petroleum traps. This study illustrates that a regional gravity analysis can be useful in determining where additional exploration can be applied in relatively unexplored basins.

The Campos Basin is a petroleum-productive, marginal sag basin along the Brazilian margin. It contains a stratigraphic sequence recording lithospheric extension and rift tectonics developing to a fully evolved post-break-up setting. We present a combined approach using subsidence analysis and basin history inversion models to make predictions on source-rock maturation. The classical uniform stretching model does not account for the observed tectonic subsidence, and so we consider the thermal and subsidence implications of incorporating different events of magmatic underplating. The post-rift is characterized by one major phase of post-rift thermal subsidence: changes in sediment supply and load, as well as halokinetic movements, cause deviations from normal thermal subsidence and relaxation. The dimensions of these events have been examined and quantified.

Our best-fit forward model is tested against measured borehole temperature (BHT) data. Results show a systematic overestimation of present-day temperatures in the shallow-water wells. Including the effects of hydrothermal convection by raising the conductivity of the top part of the stratigraphic sequence results in an almost perfect fit between predicted and measured BHTs in the shallow-water wells.

Based on these forward models, we predict that the lacustrine shales of the syn-rift Lagoa Feia Fm. could be mature to generate oil and/or gas over a larger area than previously expected. Furthermore, our results suggest that the early post-rift Macaé Fm. is locally mature to generate oil, and that the middle/late post-rift Carapebus/Ubatuba Fm. shales are likely to generate oil as well, if organic-rich facies are indeed present. In light of the most recent pre- and post-salt discoveries made near established oil fields in the Campos Basin, our results support the idea that many more such accumulations may be found.

A new methodology for robust, high-resolution correlation of reservoir sandstones in highly compactable depositional sequences is proposed. Quantitative sequential re-burial modelling has been successfully applied on real data from seven wells covering the heterogeneous fluviodeltaic Åre Formation in the Heidrun Field, offshore mid-Norway. The methodology is based on ten interpreted lithofacies classes derived from core descriptions and wireline logs signatures, in addition to interpreted sequence stratigraphic surfaces, i.e. flooding surfaces. Analysis of decompacted sedimentary columns, with emphasis on studies of shallow compaction effects tied to uniquely calculated compaction curves, has revealed several new correlatable horizons within the Åre Formation. These include laterally extensive coals and several laterally correlatable fluvial sandstones enabling a reinterpretation of parts of the Åre stratigraphy. The results from the present study demonstrate the benefits of correcting for the effects of differential compaction in well-to-well correlation of heterogeneous reservoirs comprising highly compactable sediments. The methodology outlined here has widespread applicability to other stratigraphic successions and could potentially help in the correlation of highly compacted sediments in the subsurface.

In this paper, we investigate subcritical propagation of an initially oil-filled, sub-horizontal microcrack driven by the excess fluid pressure associated with the conversion of oil to gas in a petroleum source rock under continuous burial. The crack propagation distance and propagation duration (the time required for the crack to propagate during conversion of all oil to gas), as well as the excess pressure inside the crack, are determined using a finite difference scheme that couples linear elastic fracture mechanics, oil–gas transformation kinetics and an equation of state for the gas. The effects of the source-rock temperature at the initial depth of the microcrack and fracture properties of the source rock are also considered. Our numerical results show that higher burial rates significantly reduce the crack propagation duration. However, the influence of the geothermal gradient on the propagation duration and distance is only marginal. Similar to the results for the oil-driven crack propagation during kerogen–oil conversion, the duration of gas-driven crack propagation is also governed by transformation kinetics because the subcritical crack propagation rate is much faster than the oil–gas conversion rate.

Creating earth models for deep-water appraisal and development studies is perhaps the most challenging task confronting the petroleum geologist today. Data are limited (few wells, limited core, untested seismic quality), time is limited and drilling, testing and facilities costs are very high. Uncertainty in geological characterization of the reservoir can have the greatest potential impact on project value. How can a thorough characterization of reservoir uncertainty be made based on limited data and in a timely fashion? A workflow for creation of suites of models for appraisal and development studies of deep-water reservoirs is described. The goal of the workflow is to rapidly construct suites of earth models based on limited data that capture the full range of uncertainty in reservoir characteristics and properties. After characterizing possible distributions for individual parameters, suites of earth models are built in a single step using an experimental design framework, aided by a powerful workflow manager which automates earth model construction. Earth models created using the experimental design framework are seamlessly linked to flow simulation software. Plackett–Burman, folded Plackett–Burman and Full Factorial experimental designs were used in different appraisal and development cases. Multiple experimental designs were produced by adding and modifying uncertainty parameters as additional data arrived, and ideas about the possible character of the reservoir evolved. In the appraisal case described here, 6 experimental designs were made, 388 earth models were created and studied, and 79 of those models were dynamically simulated. The process of quickly building and re-building suites of earth models using experimental designs to address changing perceptions and concerns about uncertainty in reservoir character is termed here ‘procycling’. Procycling is complementary to experimental design studies, in that multiple experimental designs are employed over time: procycling focuses on changes in predictions made by individual experimental design studies. The results of procycling are not necessarily to change the perception of uncertainty (for example the range of possible outcomes), but to anchor what the limits of uncertainty are and what the most important uncertainties are with the given data.

A new well testing response from lateral cross flow within layers is described. The response occurs when there is extremely low effective vertical permeability in the system at the larger scale. Low vertical permeability actually accentuates the layering and reduces vertical cross flow whilst enhancing lateral cross flow from within-layer heterogeneities. The response is investigated using numerical simulation of flow in end-member models of complex and geologically realistic architecture in high net-to-gross fluvial systems. This ‘ramp’ response is shown to form one member of a family of well test pressure transient responses. The other members of the family include previously-described negative geoskin and geochoke. The use of well test data to characterize these particular types of layered fluvial reservoirs is an important step in the static-dynamic integration of geological and reservoir engineering models.

Many of the original muddy marine sediments that have compacted to become gas shale could have been in a depositional environment suitable for the formation of natural gas hydrate (NGH), which concentrates gas by a factor of 164 (at STP). Dispersed biogenic NGH in fine-grained continental slope sediments today occurs in sections as thick as 250 m and contains enormous amounts of methane. Concentrated NGH can completely fill porosity in more permeable sediments. Formation of NGH in the early diagenetic history of shale gas sediments may have been the first step in the gas concentration process. NGH that formed in ancient gas shale sediments could have persisted and held the natural gas in place during lithification so long as hydrate remained stable. It is possible that the concentrated gas was held in place until the packing of the clay minerals effectively reduced permeability to a point that the gas released from naturally converting hydrate could not migrate easily. Because NGH creates open porosity upon conversion, a very large part of this gas could have been trapped in the shales before dissociation of the NGH to its component water and gas was completed. An implication for shale gas exploration is that high gas concentrations may not be confined to organic-rich shales but may also be found in any shales that once contained substantial gas hydrates. These include grey shales with lower organic content and more siliceous shales, which respond well to fracking.

Grids used for flow simulation are often at a much coarser scale than that of grids for geological modelling due to computational demand. Unstructured grids offer increased flexibility for the flow grid design; however, solving the flow equations and upscaling from high resolution geological grids to the coarse flow grid is more complex than using coarse regular grids. The multipoint flux approximation (MPFA) is one technique applied to discretize the flow equations on unstructured grids. This paper develops an upscaling technique that uses the MPFA method to solve the flow equations on the fine- and coarse-scale grids. Unlike most cases where the fine-scale grid is regular or structured, this work utilizes a high resolution triangular grid that conforms to the coarse-scale grid. The triangular grid is generated using the coarse-scale interaction regions as constraints. Upscaling leads to transmissibility matrices of the coarse-scale interaction regions. Two different types of local boundary conditions for the MPFA upscaling approach are developed, including linear varying pressures and pressures computed by solving the flow equations around the element boundary. The method is tested using flow simulation on several cases. Results are comparable with flow using a high resolution regular grid.