Enhancing Hydrocarbon Estimates in a Deepwater Turbidite Sequence
Authors: Nelson Rick , Clavaud Jean-Baptiste , Guru Udit Kumar , Wang Hanming 
Published: February 2015

Thinly laminated formations can be significant hydrocarbon reservoirs, particularly in turbiditic and fluvial environments. However, often such formations exhibit low resistivities when measured with conventional resistivity tools. These are known in the literature as classical low-resistivity pay and often are anisotropic in resistivity. Over the past decade, studies have shown that electrical anisotropy greater than three in these layered formations is caused by thin water-bearing beds, such as shale layers, alternating with the oil sands. Initial laboratory studies have shown that alternating variations in water saturation is a major contributor to resistivity anisotropy, but detailed laboratory research is needed before this can be generalized to all shaly-sand reservoirs.

Since their introduction in the early 1950s, wireline induction tools measured mainly the horizontal resistivity (Rh). Consequently, when confronted with a potential reservoir containing thinly laminated sand/shale sequences, it was a challenging task to decipher the thin hydrocarbon-bearing sands in the apparent low-resistivity hydrocarbon-bearing layer. The low-resistivity reading had to be corrected, usually by adding a shale-contribution term using one of the many published equations. More recently, technologies have been developed to measure the vertical dimension of this resistivity problem, or the vertical resistivity (Rv), which can be evaluated using three tools: logging-while-drilling resistivity tools when the apparent angle between the tool and the formation is high; joint inversion of array laterolog and array-induction; and the triaxial induction tool, the focus of this article.

North Africa Case Study

In a North Africa well, array-induction, nuclear magnetic-resonance (NMR), and nuclear (density-neutron and gamma ray) tools were run initially. The target interval was a deepwater turbiditic levee/overbank deposit, consisting of highly organized thin layers of high-quality gas-bearing sands. Layer thicknesses ranged from almost a meter to less than a centimeter, with most of the layers being in the centimeter range. The logged well encountered two gas-filled channel systems separated by a pressure barrier (Fig. 1). The lower system was a more proximal levee/overbank facies, of which the reservoir portion was made up of organized layers of high-quality coarse-grained sand interbedded with shales and some mudstones. Conversely, the upper system consisted of a levee/overbank facies having thin layers of finer-grained sands with shales and small amounts of mud.

Regular array-induction logs did not show noticeable invasion profile in this formation. The porosity (including clay- bound water) was 35%, and the effective porosity (without clay-bound water) was 25%. The separation between density and neutron showed a high amount of shale except in three places (X150–X155 ft, X188–X192 ft, and X268–X271 ft), where the cross-over between density and neutron indicated gas, also confirmed there by the higher resistivity measured by the regular induction log (Fig. 2). From the deep array-induction log, water saturation (Sw) was computed with a dual-water equation to correct for clay conductivity. The data analysis, based on the array-induction tool and a dual-water shaly-sand approach, led to unreasonably high water saturations, with all resulting Sw values being close to unity, except in the three higher resistivity depths.