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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Wireline Geochemical Log Analysis
of Thin Bed Reservoirs
Gulf of Mexico
By
Reliable detection of low contrast pay sections in offshore Pliocene strata has been facilitated by the increased resistivity associated with the oil and gas over the associated shales and water zones. However, the contrast in resistivity is often more difficult to detect in some reservoirs. Consequently, low-contrast reservoir horizons may be overlooked.
Detailed lithological studies reveal that in many cases the subtle low resistivity reservoirs consist of thin alternating laminae or beds of sandstone and shale. Core plugs usually indicate the sandstone lenses have low water saturations, high resistivity and good porosity. Associated shaly layers have limited porosity and permeability, high water saturations and low resistivity. Proper evaluation of porosity and fluid saturation is difficult as the vertical resolution of many wireline services is too large to resolve the true properties of the thinly layered hydrocarbon productive sandstones. The wireline log response reflects the average water saturation and porosity of both the productive sand and the nonproductive shale layers.
Evaluation of thinly layered reservoir rocks is facilitated by an integration of wireline log data and data derived from detailed evaluation of core samples. This talk addresses one method of integrating core evaluation with geochemical wireline log measurements.
The objective is to compare wireline geochemical
methods and traditional methods to determine if the
geochemical evaluation has any advantages over traditional
evaluation for thinly bedded reservoirs. In the example well,
the wireline geochemical method measures higher hydrocarbon
volume in a thin-bed reservoir. The higher hydrocarbon
volume results from higher computed effective
porosity from the geochemical method. Porosity is verified
from core measurement, but since only sidewall cores
rather than whole-core plugs are available, porosity agreement
is considered to be a qualitative verification
. Quantitatively,
the wireline geochemical method is used to calculate
and compare to core measurement, values of cation
exchange capacity from a knowledge of clay types and
abundances.
The wireline geochemical method uses "a geochemical
model
" that has been calibrated with several hundred core
plug measurements of elements, minerals, porosity, cation
exchange capacity, grain size, and permeability from previous
studies. The mineralogy defined by the geochemical
model
provides the basis for defining tool responses needed
for interpretation of the solid portion of the rock. Additional
wireline measurements provide information for the fluid
portion of the rock. Combining the fluid analysis and the
mineralogy provides the calculation of hydrocarbon
reserves. The wireline geochemical method requires one
more logging run which collects over twice as much data as
traditional logging runs.
From a comparison of the wireline geochemical method with traditional methods, we conclude that the mineralogy derived from the elemental log measurements provides clay fractions that are confirmed by cores thus contributing to the evaluation of thinly bedded reservoirs.
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