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Discrimination of lithofacies in tight gas reservoir using field-specific rock physics modeling scheme. A case study from a mature field of middle Indus Basin, Pakistan

Durrani Muhammad Zahid Afzal, Rahman Syed Atif, Talib Maryam, Sarosh Bakhtawer

Acta Geophysica

2023

Vol. 71, no. 6

2763--2780

Consistency in the petrophysical and elastic properties is very critical for the characterization in low to intermediate (tight) porosity sandstone reservoirs. In this case study, we have applied an iterative and integrated workflow that provided consistency between the petrophysical and elastic properties using rock physics modeling scheme for the quantitative characterization of the low to intermediate porosity reservoir of Cretaceous (Pab) sandstone reservoir of the mature field in middle Indus basin of Pakistan. Before petrophysics and rock physics modeling (RPM), the well logs data quality is assessed and conditioned. We employed an inclusion-based rock physics model to estimate elastic (P-wave, S-wave, and density) properties by accounting for the effect of mineralogy using pore geometry (pore aspect ratio) variation. The RPM provided consistent elastic and petrophysical properties when compared with measured logs and improved lithofacies understanding in the tight gas reservoir. Finally, modeled elastic properties and lithofacies are assessed and characterized in a rock physics template (RPT) using an effective medium theory. The successful application of the integrated workflow exterminated the well log interpretation uncertainty by providing a consistency between the petrophysics and RPM, which can be extended for improved reservoir characterization and prospect evaluation across other areas with similar geological trends and reservoir distribution.

Quantitative characterization of unconventional (tight) hydrocarbon reservoir by integrating rock physics analysis and seismic inversion: a case study from the Lower Indus Basin of Pakistan

Talib Maryam, Durrani Muhammad Zahid Afzal, Palekar Arshad Hussain, Sarosh Bakhtawer, Rahman Syed Atif

Acta Geophysica

2022

Vol. 70, no 6

2715--2731

The paper demonstrates a successful application of Bayesian classification method to accurately predict petrophysical properties and lithofacies classification in the deep unconventional (tight gas) hydrocarbon resource potential of early Cretaceous in the Lower Indus Basin of Pakistan. To explore the true potential for exploration and development phases, we quantitatively characterized the tight gas reservoir based on an integrated methodology using the Bayesian approach constraint with rock physics analysis which utilized deterministic petrophysical results from a well information to extract the desired lithofacies at seismic scale. The employed methodology relied on stepwise sequential integration of all available data through petrophysical, rock physics analysis and seismic inversion technique. Simultaneous inversion approach is used to invert elastic properties for reservoir interpretation. Seismic-based petrophysical properties are predicted using regression analysis by establishing a functional relationship between well logs for Sembar formation. The rock physics template (acoustic impedance versus Vs/ Vs ratio) model helped to differentiate lithological units of sand and shale in the well. Three lithofacies (HC sands, shale and shalier sand) are properly classified in rock physics template, and their probabilities are accurately defined using Bayes’ theorem. Finally, estimated lithofacies and hydrocarbon probability map from the Bayesian approach are meticulously validated from well data. The quantitative seismic reservoir characterization study provided important support for the unconventional prospect evaluation and hydrocarbon reserve estimations necessary to delineate unexplored parts which could prove helpful in effectively planning for the horizontal well placement and optimal reservoir development.