A Comprehensive Approach to Forecast Reservoir Quality in Thrust-Faulted Domains: Integrating Structural Analyses, Charge Modeling and Reservoir Modeling

Gary P. A. Muscio, Carlos Rivero, and Andrew R. Thomas
Chevron Energy Technology Company, Houston, TX

Recent efforts in the numerical integration of structural analyses with classical charge modeling (maturity and fluid flow) illustrate the impact of structural evolution on temperature distribution in time and space in an oil basin. This in turns affects the timing of generation, expulsion, and migration of hydrocarbons from source rocks, and the physical and chemical parameters that enhance or degrade reservoir rocks. In this paper, we focus on an integrated approach that involves structural evolution and timing of source rock generation with reservoir quality prediction in sub-thrust plays. The integrated numerical approach has the potential to improve our understanding of the thermal and effective stress histories which control the timing between structural pulses that define trap generation and trap enhancement, and phases of source rock maturation and fluid flow. The accurate definition of the temperature and effective stress histories is also a critical input for predictive reservoir quality modeling, as both histories influence textural and diagenetic conditions in these intervals.

While basin modeling tools historically have simulated structural burial and compaction as predominantly vertical processes, the effect of lateral tectonic movement on the temperature and effective stress histories of oil basins with a strong transition of structural styles (i.e. extensional and contractional provinces in gravitational fold–thrust belts) has not been addressed adequately until recently.

Here we present an integrated approach to model selected 2D transects across a variety of structural styles in sub-thrust locations of an petroleum-bearing marine fold-and-thrust belt. Each transect was fully restored from top to bottom at time steps defined by a regional sequence stratigraphic framework. For each time step, the 2D sections were first decompacted using a regional porosity-loss trend developed from well data. The 2D sections were then restored and corrected isostatically to ensure compliance with structural and geological principles before final submission for thermal and fluid flow modeling. A critical step in the workflow was the inclusion of a detailed, log-based reservoir model at the beginning of the process, in order to constrain the lateral and vertical distribution of sandy intervals and their properties. Finally, effective stress and temperature histories were extracted from selected reservoir intervals of the 2D model and used as input for reservoir quality modeling. The forward modeling of reservoir properties in various locations within the 2D lines was assisted using calibrated rock properties from a variety of well locations in the study area. Reservoir properties were predicted in hanging wall and footwall locations, thereby reducing uncertainty in thrust plays.

The results show that sequentially restored paleosections based on rigorous structural analyses generate trap geometries and trap evolutions that are consistent with reasonable uplift (structural relief) and burial histories, especially in contractional fold-and-thrust belts (Fig. 1). In contrast, tool-driven structural approaches, which are mostly based on vertical restoration, introduce geometric artifacts that compromise thermal and burial histories. Sequential structural restorations provide better constraints on trap geometries in footwall settings, resulting in an improved representation of the timing between trap generation and thermal history for sub-thrust locations. This also applies to the modeling of effective stress which can be misrepresented using traditional approaches that lack the benefits of appropriate structural restorations. The cumulative effect of these misrepresentations can have a significant impact on reservoir quality prediction efforts and therefore in the assessments of economic basement. The results of this study also highlight the importance of applying an integrated approach to reduce risks associated with appraising sub-thrust locations, and to ensure that basin models generates outputs that are constrained and validated by structural analyses.

Figure 1. Thermal and compaction modeling in fold-thrust belts provinces without appropriate structural analysis lead to incorrect restored trap geometries and trap evolution. This in turn affects the estimation of thermal and mechanical parameters relevant for charge and flow modeling and reservoir quality prediction.

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.