Neil R. Braunsdorf

Sandstone diagenesis and reservoir quality prediction: Models, myths, and reality

Models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration. Not all of these are equally supported by quantitative data, observations, and rigorous hypothesis testing. Simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones. Calibrated numerical models that simulate compaction and quartz cementation, when linked to basin models, have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture, composition, and quartz surface area is available. Analysis of global, regional, and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality. (1) The effectiveness of grain coatings on quartz grains (e.g., chlorite, microquartz) as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results. (2) Vertical effective stress, although a fundamental factor in compaction, cannot be used alone as an accurate predictor of porosity for lithified sandstones. (3) Secondary porosity related to dissolution of framework grains and/or cements is most commonly volumetrically minor (2%). Exceptions are rare and not easily predicted with current models. (4) The hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model. (5) Heat-flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure, thereby slowing the rate of quartz cementation in some subsalt sands.

Seismic Petrophysics for Clean Sandstones: Integrated Interrogation of Lab- and Well-based Data for Improved Rock Physics Modeling

Rock physics relationships are an essential element in the evaluation and modeling of seismic attributes for hydrocarbon exploration. Integration of laband wellbased data is essential in the development of rock physics models with predictive capability. Our integrated workflow for seismic petrophysics includes detailed petrologic characterization, incorporation of fundamental rock properties controls, rigorous petrophysical evaluation, and the use of a core-calibrated rock physics model. Results from a recent integrated study in the Gulf of Mexico yield further validation of the modulus-based model (Kittridge et al. 2004) and provide essential insights into the transition to consolidated sandstone behavior. The new modulus model is different from existing relationships for clean sandstone and we suggest possible causes.

Velocity-based exploration for basin centered gas accumulations: a paradigm revised.

Summary This investigation was aimed at improving the understanding of P-wave velocity behavior in Tight Gas Sands (TGS) and associated rocks. The specific goal of this study was a determination of the cause(s) of observed Pwave velocity slowdowns often associated with basincentered gas accumulations. The study focused on Cretaceous-age rocks in the Greater Green River Basin (GGRB), Wyoming. Study results indicate that variations in the sonic velocity log and seismic interval velocity responses are related predominantly to lithology. In contrast, gas saturation and abnormal pressures have a comparatively small impact on P-wave velocities at both the regional and field scales. We were unable to confirm the findings of previous studies purporting that seismic velocity could be used to detect abnormal pressures or gas-saturated rocks in the GGRB and call into question the use of velocity anomalies as an exploration tool in the search for gas accumulations in the GGRB. The significant influence of lithology on P-wave velocity response suggests we may apply seismic velocities and related attributes, such as acoustic impedance, to lithology identification in the GGRB.