Influence of Clay Diagenesis on Shale Velocities and Fluid-Pressure
Authors: Lahann R.W. , Lahann W. , McCarty D.K. , Hsieh J.C.C. 
Published: October 2008

Fluid pressure in the pore system of sedimentary rocks is an important parameter to the exploration and production of petroleum. Compaction disequilibrium is considered by many by many workers to be the dominant pressure-generating mechanism. Clay mineral diagenesis (particularly the smectite to illite reaction) is recognized as a potential contributor to pore fluid pressure, but there have been few systematic studies which integrate clay reaction extent with pressure and log responses. This study examines samples from two basins with different shale mineralogy by detailed characterization of mudstone drill cuttings by X-ray diffraction, and compares these data with pore pressure information, bulk density and sonic velocity obtained from standard wireline well logging.

Samples from Well A, with unreactive illite-smectite (I-S) yielded a single stress/density and single stress/velocity relationship. Unreactive I-S also yielded a simple velocity/density relationship where samples from a velocity reversal zone have the same sonic/density relationship as do samples from shallow zones in the well.

Well B, with a reactive I-S suite, has complex stress/velocity, stress/density and velocity/density relationships. Open system illitization induced a more rapid velocity increase, relative to the rate of bulk density increase. Illitization with unloading induced velocity reduction while allowing bulk density to increase. The effect of illitization on density/stress relationships appears to be most dramatic during the latter stages of illitization.

Introduction

Recent studies on overpressure mechanisms have mostly emphasized the role of compaction disequilibrium as the most significant process in generating overpressure (e.g., Osborne and Swarbrick, 1997; Audet, 1995; Hart et al., 1995). The role of clay diagenesis as a pressure mechanism has been minimized and is considered to be to be only locally important. Some studies on clay diagenesis/pressure generation have emphasized the possible volume change associated with the clay reaction (Osborne and Swarbrick, 1997). More recent work by Lahann (1998, 2001) suggests that the primary role that clay diagenesis plays is to change the compaction response of the shale. In this model, the clayderived water released from hydrated interlayer cations in expandable mixed-layer illite-smectite (I-S) during the illitization reaction is a secondary pressure source and the change in the water volume is neglected.

In addition to the role of the illitization reaction in fluid pressure generation, the effect of illitization on the petrophysical properties of shales and mudstones is poorly understood. Deeply buried shales from the Gulf of Mexico shelf, which presumably have undergone illitization, often slow sonic velocities (for their density) relative to shales with a higher smectite layer content (expandability) in the I-S. However, since illitized zones are often overpressured, it is difficult to separate a possible change (relative sonic slowing) due to illitization, from a slowing associated with unloading (Bowers, 1994; Bowers, 1998).

The objective of this study is to integrate detailed mixedlayer clay mineral characterization information with wireline log and pressure data. Two wells are examined which exhibit distinctly different mineralogical profiles and which have different depth trends for sonic velocity and bulk density.