ABSTRACT

Conventional petrophysical evaluation techniques are unreliable to assess individual bed properties in laminated depositional sequences with beds thinner than the vertical resolution of standard logging tools. The main cause of this limitation is that well logs average formation properties across multiple interbedded intervals. Commonly used solutions are limited to shaly sandstone models to account for either presence of graincoating clay in sandstones or laminated shale-sandstone systems. Other solutions rely on volumetric techniques which require subjective interpretation of volumetric concentration of shale and total porosity. Likewise, it is typically assumed that both sandstone and shale properties remain constant within siliciclastic reservoirs, which is not always the case in heterolithic bedding or in laminated sequences with strong diagenetic alterations.

To address this technical challenge, we introduce an inversion workflow that reproduces measurements via analogues of thinly-laminated reservoirs. We use a Markov-Chain Monte Carlo inversion algorithm to generate independent realizations of each petrophysical property. All petrophysical properties are combined to estimate probability histograms rather than attempting to obtain a single value for each petrophysical property.

The method is applied to a deepwater heterolitic clastic sequence of grain-coating clay sandstones where bed thickness varies from 3 to 4 inches. In addition to conventional well logs, high-resolution borehole images are used to detect bed boundaries. The statistical method is used to estimate total porosity, water saturation, and permeability based on the earth-model-derived properties. Finally, net-to-gross and hydrocarbon pore volume are estimated using the calculated statistical properties.

Compared to conventional interpretation procedures, the formation evaluation method developed in this paper enables the incorporation of non-constant matrix and shale properties in the sandstone-shale laminated sequence, and estimates individual layer properties and their uncertainties, thereby reducing subjectivity in the interpretation of static and dynamic petrophysical properties of heterolithic clastic sedimentary sequences.

INTRODUCTION

Complex depositional systems generate spatially heterogeneous reservoirs with petrophysical properties difficult to quantify. The main challenge in studying thinly-laminated formations arises when the vertical resolution of traditional logging devices is smaller than the thickness of the beds. Conventional logging tools average formation properties in such sedimentary systems. In addition, in these environments the electric conductivity of shale dominates the response of conventional electric resistivity logs (Shray and Borbas, 2001). Hence, a typical petrophysical assessment of thinly-bedded shaly-sandstone systems leads to underestimation of hydrocarbon saturation (Pritchard et al., 2003) and poorly characterized permeability.

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