High Resolution Sequence Stratigraphy Analysis in a Carbonate Reservoir from Borehole Resistivity Image: a Case Study from Tarim Basin, West China
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Abstract Formation heterogeneity due to fractures, vugs, and mixed lithologies complicates the characterization of carbonate reservoirs. The lithology distribution is controlled by multiple factors, such as sediment source, depositional environment, and diagenesis. In addition, fracture development is influenced by lithology, burial depth, local structure, and far-field stress. High-resolution sequence stratigraphy is one of the advanced methods that can be used to solve the lithology challenge. The method combines core analysis, conventional logs, outcrop studies, and seismic data. However, the analysis results are frequently constrained by the low resolution of the seismic and conventional log data and by limited core data. A new workflow for high-resolution sequence stratigraphy analysis integrates borehole resistivity images with seismic, log, and core data. First, the borehole resistivity images are compared with core data, and the depositional facies are identified from calibrated resistivity image data combined with multiple-domain data. Second, sequence stratigraphic surfaces are identified from seismic and image data and the thicknesses of crossbedding and sequence cycles are used to classify the strata stacking patterns. Finally, the distribution of depositional environments within a sequence stratigraphy framework is analyzed by integrating the sequence stratigraphy patterns with seismic attribute maps and petrophysical log interpretation to predict the sweet spot. This new approach was implemented in Block A8 of the Tazhong uplift in the Tarim basin. Six different depositional facies were identified from the core data from three wells and applied to an additional four wells and to noncored intervals. Isopach maps of the first long term sequence cycle were used to estimate the size of buildups (reef, mound) and predict the vug distribution. A recently drilled well confirmed the analysis results. This workflow can be applied to similar thick carbonate reservoirs in the shoal-reef margin of a carbonate platform. Introduction The Tarim basin has three main belts: the Mesozoic and Cenozoic foreland, the Tabei uplift, and the Tazhong uplift (Xianming Xiao et al. 2004). The Tazhong uplift, which has an area of 30 000 km2 is the most complex and difficult area for exploration. The Tazhong-1 slope-break zone extends 200 km from east to west adjacent to a major thrust fault zone in Lower to Upper Ordovician carbonate rocks. Much of the oil and gas discovered in the area occurs in the Upper Ordovician Lianglitage formation. A complex shelf depositional system developed during deposition of the Lianglitage formation. At the same time, the continental shelf margin subfacies along the Tazhong I faulted slope-break zone was developed, and high-energy reef, shoal, and mound facies developed. Within the Tazhong shelf, tidal flat, gently sloping shelf shoal, shelf wash, and mound subfacies developed. The study area, block A8, is located in the western part of the Tazhong I slope break. The first well was drilled in 2005. Eight wells have been drilled, and three of these show good production (Fig. 1). One more appraisal well was recently drilled.Keywords:
Sequence Stratigraphy
Lithology
Isopach map
Outcrop
Petrophysics
ABSTRACT Quality, availability and consistency of the measured and interpreted well log data are essential in the seismic reservoir characterization methods, and seismic petrophysics is the recommended workflow to achieve data consistency between logs and seismic domains. This paper uses seismic petrophysics workflow to improve well logs and pore geometry interpretations for an oil carbonate reservoir in the Fahliyan Formation in the southwest of Iran. The petrophysical interpreted well logs, rock physics and well‐to‐seismic tie analysis are integrated into the proposed workflow. Our implementation incorporates revising petrophysical well log interpretations and updating pore geometry characteristics to obtain a better well‐tie quality. We first propose an improved pore‐type characterization approach based on both P‐ and S‐wave velocities for quantifying pore geometry. Then, seismic logs are estimated accordingly, and the results are used in the well‐to‐seismic analysis. The quality of the well‐tie is improved, furthermore, by iterating on the petrophysical interpreted well logs as well as the calculated pore geometries. For the intervals with high‐quality data, our workflow improves the consistency between the results of measured and modelled seismic logs. For the intervals with problematic well logs, the application of our proposed workflow results in the successful replacement of the poor data and subsequently leads to an improved wavelet estimation and well‐tie results. In both cases, a higher quantification of pore geometries is achieved, which in turn is confirmed by the core images and formation micro‐imager analysis.
Petrophysics
Seismic to simulation
Characterization
Formation evaluation
Seismic attribute
Environmental geology
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Summary form only given. Two of the most important answer products of the traditional petrophysical processing are porosity and hydrocarbon saturation, which are usually presented as logging curves representing volumetric averages of the corresponding petrophysical properties only. In general, those petrophysical properties are distributed inhomogeneously around the wellbore. For cases, such as the existence of mud-filtrate invasion, those logging curves could be far off from the true values, thus leading to erroneous formation evaluation. As a result, for accurate formation evaluation, it is preferable to obtain the distributions of those petrophysical properties around the wellbore. Moreover, in general, multi-physics measurements are required to obtain both porosity and fluid saturations. Advancements in modern well logging technology have made this possible. For example, the tri-axial induction tool and sonic tool are capable of making three-dimensional induction and sonic measurements at multiple depths of investigation at multi-frequencies. In other words, these tools can make measurements in a wide spatial and spectral coverage. In addition, these tools can also see past the altered zone by mud-filtrate invasion and provides measurements for the unaltered zone. The availability of these measurements makes it possible to derive porosity and fluid saturation images around the borehole, which could result in enhanced porosity and hydrocarbon saturation estimates, thus leading to improved reservoir characterization. In this paper, an inversion method is developed for directly obtaining porosity and fluid saturation distributions by simultaneously inverting borehole induction and sonic measurements via petrophysical relationships. The inversion is accomplished by fitting both induction and sonic measurements in an automated manner using the state of the art regularization technique and modern parallel computation techniques. We present several examples covering situations like mud-filtrate invasion, fault, fractures, as well as water flooding, which show the proposed joint inversion reduces the non-uniqueness of determining porosity and fluid saturation distributions, which cannot be achieved with inversions using sonic data or induction data only. We show that the induction measurements and acoustic measurements can benefit each other so that improved porosity and fluid saturations distributions can be obtained. Induction measurements can extend the spatial reach of the acoustic measurements while the acoustic measurements can better condition the induction measurements in such a way that the saturation images can be obtained with enhanced resolution.
Petrophysics
Saturation (graph theory)
Sonic logging
Wellbore
Hydrocarbon exploration
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Logging data from gamma ray, neutron-porosity, density, resistivity etc, are helpful to perform the qualitative and quantitative evaluation of the drilled wells. Parameters like water resistivity, porosity, water saturation etc, can be estimated by using the logging data. The Archie equation based method was a suitable method for all zones, due to the fact that no zone of interesting showed volume of shale bigger that 50%. In order to determine the water saturation in the formation, formula expressing the relationship between the true resistivity (Rt) and the formation parameters affecting the resistivity was used. The picket plot was a suitable method to estimate the water resistivity. Could be concluded that when no pressure data is available, as in the case of JOIA well, the logging curves, neutron and density, by plotting them in the neutron –density crossplot (qualitative evaluation), can be used to estimate the type of fluid in the formation. Although the results were not as accurate as from the pressure or core data. The neutron-density crossplot is indicating existence of oil in JOIA well reservoir.To have good permeability estimation, it’s necessary to have enough core data so that the regression line can be drawn. This paper presented applications of the petrophysical theories for qualitative and quantitative evaluation of two explorations wells of offshore Angola that will be called EKS and JOIA well respectively, with help of a petrophysical software called Techlog.
Petrophysics
Formation evaluation
Saturation (graph theory)
Water saturation
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