Отправлено: 01.06.18 05:01. Заголовок: Сходная тема интересует бывшего кубанского петрофизика

The Problem With Silt in Low-Resistivity Low-Contrast (LRLC) Pay Reservoirs1
Alexander Belevich and Adriaan A. Bal

Clastic laminated reservoirs have historically posed
dif¿ culties in formation evaluation. Dif¿ culties 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 Southeast Asian (SEA) basins the abundance of silt in reservoir and
nonreservoir 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 ef¿ cacy 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, deconvolution, and digital core imaging
analysis.

Bulk rock (or volumetric) approaches generally use Thomas-Stieber, multicomponent 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 nonlinear “boomerang” shape.

Resistivity anisotropy, i.e., the vertical to 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
multicomponent resistivity results.

One very silty to ¿ negrained sand reservoir in Vietnam displays anisotropy due
to grain-size variation on a very ¿ ne level.

In this example, the relevance of shale laminar volume is questioned and can
only be justi¿ ed with detailed core studies.

It is, however, argued that reliable identi¿ cation of hydrocarbon-bearing
silt-rich sequences is only possible with multicomponent
resistivity data. In addition, quanti¿ cation 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