Clastic laminated reservoirs have historically posed difficulties in formation evaluation. Difficulties are largely due to convoluted log responses which preclude accurate assessment of key petrophysical properties such as thin sand bed porosity and water saturation. In South East Asian (SEA) basins the abundance of silt in reservoir and non-reservoir rocks adds another layer of complexity and directly affects the design of appropriate data acquisition programs. This paper describes the silty thin bed problem by assessing the efficacy and uncertainties of various log measurements to arrive at the correct petrophysical solution.
A review of rock physics literature is presented to highlight the underlying reasons for log behavior in silty facies. Generally, laminated rocks are evaluated from two different approaches: (1) high-resolution or (2) bulk rock volume. High-resolution approaches include borehole image logs, de-convolution, and digital core imaging analysis. Bulk rock (or volumetric) approaches generally use Thomas-Stieber, multi-component resistivity, and nuclear magnetic resonance (NMR) techniques. Three wells drilled in different sedimentary basins in SEA were selected to demonstrate the theory, challenges, and pitfalls of the most common approaches and techniques.
The Thomas-Stieber approach is often regarded as the most suitable for a binary sand-shale system and if conditional assumptions are met, results in a linear trend from which net-to-gross can be calculated. Adding a third component, such as silt, violates the assumptions and distorts this trend into a non-linear "boomerang" shape.
Resistivity anisotropy, i.e., vertical/horizontal resistivity ratio (Rv/Rh), provides further necessary input for accurate formation evaluation in laminated sand-silt-clay systems. Vertical resistivity is a key measurement as it is very sensitive to hydrocarbons in thinly laminated sands. Additional information, like borehole image and NMR data, contribute to reducing net-to-gross uncertainty and understanding the reservoir geometry. Where available, the saturation height function results are compared to multi-component resistivity results. One very silty to fine-grained sand reservoir in Vietnam, displays anisotropy due to grain-size variation on a very fine level. In this example, the relevance of shale laminar volume is questioned and can only be justified with detailed core studies. It is, however, argued that reliable identification of hydrocarbon-bearing silt-rich sequences is only possible with multi-component resistivity data. In addition, quantification of sand lamina resistivity, Rsand, is possible in silty sands with variable amounts of irreducible water.
Although many papers discuss the thin-bed formation evaluation problem, very few publications address issues related to laminated sand-silt-clay reservoirs. This paper partly addresses this literature scarcity.
Alexander Belevich graduated from Kuban State University in Russia with University Degree in Formation Evaluation.
Alexander has worked in various locations in Russia (Siberia, above the Arctic circle and Moscow). Later moved to Vietnam for a 4 year stint and has been working in Malaysia for the last 3 years as Asia Pacific regional petrophysics advisor, doing field studies, designing and supporting acquisition of wireline & LWD open hole programs. Has special interest in evaluation of LRLC reservoirs, interpretation of NMR and 3D Resistivity logs, log and core data integration.