Summary

Characterization of the elastic properties of the Middle Bakken Siltsone demonstrates the need to account for porestiffness effects in a seismic-based assessment of this resource play. Well data from this oil-producing interval show inconsistencies between density and velocity measurements. More specifically, the density can be modeled accurately as a function of mineralogy, porosity, and pore fluid. This same set of parameters, when used in theoretical bounds for elastic properties, over predict the bulk modulus and to some extent the shear modulus. Fracturing is the immediate choice to explain this discrepancy, and modeling results demonstrated that stiffness associated with crack-shaped pores in a lowfrequency parameterization possibly explain the bulk modulus measurements. Accordingly, it may be important to include pore-stiffness effects when predicting productive zones in the Middle Bakken from surface seismic data.

Introduction

Understanding the elastic response of unconventional resources from surface seismic data is becoming increasingly important on the North American continent. One resource play of particular interest is the Bakken formation in the Williston Basin. Much of the knowledge about the productivity of this oil-producing interval comes from drilling and well-log information. It has been suspected that the Bakken formation produces commercially in zones where hydraulic fractures link natural fracture sets (Cramer, 1992). Locating these zones of optimal production prior to drilling would help to lower the cost of exploration of the Bakken formation The work presented here is an analysis of well data from a rock-physics standpoint with the intent of linking the reservoir and elastic properties of the Middle Bakken Siltstone (the producing unit). Tasks include modeling the measured density and elastic moduli in the Middle Bakken using theoretical bounds for different lithologies and assessing inconsistencies between the density and moduli measurements. Modeling results show that the properties that most significantly affect the seismic response of the Middle Bakken are the composition, pore shape, and pore stiffness. Some combinations of these parameters predict low effective bulk modulus values for high-density rock, which helps to explain the apparent inconsistencies between the density and velocity data.

Modeling Methodology

The Bakken Formation consists of three units: the Upper and Lower Shale units and the Middle Bakken Siltstone. Well data (Figure 1) used for this study come from the Sanish Field. The Middle Bakken lies from 3168–3183 m, in between the two shale units, noticeable in the gamma ray ( GR ), bulk density ( b ? ), and P- ( P V ) and S-wave ( S V ) velocity logs. This approximately 25-m-thick unit includes variable water saturation ( W S ) and total porosity (f ) from 3–8%. Figure 2 (middle) demonstrates that the lines for the upper bound shear modulus slightly overestimate the data. No shear bound lower modulus exists for a fluid and solid mixture. In terms of the bulk modulus, the data fall within the bounds, but the upper bound overestimates the data significantly (Figure 2, bottom).

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