Rock Physics Modeling and Seismic Interpretation to Estimate Shally Cemented Zone in Carbonate Reservoir Rock

Carbonate rock are important hydrocarbon reservoir rocks with complex texture and petrophysical properties (porosity and permeability). These complexities make the prediction reservoir characteristics (e.g. porosity and permeability) from their seismic properties more difficult. The goal of this paper are to understanding the relationship of physical properties and to see the signature carbonate initial rock and shally-carbonate rock from the reservoir. To understand the relationship between the seismic, petrophysical and geological properties, we used rock physics modeling from ultrasonic P- and S- wave velocity that measured from log data. The measurements obtained from carbonate reservoir field (gas production). X-ray diffraction and scanning electron microscope studies shown the reservoir rock are contain wackestone-packstone content. Effective medium theory to rock physics modeling are using Voigt, Reuss, and Hill.  It is shown the elastic moduly proposionally decrease with increasing porosity. Elastic properties and wave velocity are decreasing proporsionally with increasing porosity and shally cemented on the carbonate rock give higher elastic properties than initial carbonate non-cemented. Rock physics modeling can separated zones which rich of shale and less of shale.


Int r oduct i on
Reservoir rocks possess certain characteristic w hich can be identified by several physical param eters. Among many of the physical param eters are the porosity and permeability. According to [7], the characterization is a process of elaborating its characteristic qualitatively and quantitatively by using the available data. The characteristics provided as information regarding physical param eters from reservoir rock is very essential to understand the reservoir better.
In this study, there w ill be rock physics modeling to differentiate zones w hich rich of shale and slight of shale. The study areas w as located in Java Sea (Figure 1). The purpose of this study is to : (1) know ing the characteristics of elastic rock param ameter; (2) predicting the mineral-rich shale zone as cement and a mineral-slight shale zone; (3) interpreting the existence of shale gas reservoir in the study area.

Dat a and Basi c Theor y
Complex carbonate rocks is very difficult to be modeled. In addition to the types of minerals constituent of rock complex, carbonate rocks also have a variety of complex pore geometry. The only w ay to make rock physics modeling w as using effective medium theory. Illustration simplification model of carbonate rocks are show n by Figure 2. Lithology reservoir carbonate in research area w as a carbonate w ackestonemudstone w hich int erspersed w ith mineral dolomite and calcite.
The simple bounds for an isot ropic linear elastic composite, defined as giving the narrow est possible range w ithout specifying anything about the geometries of the constituents, are t he simple effectif medium theory is the Voigt bound (illustrated by Figure 1). The Voigt (M V ) and Reuss (M R ) bound calculated effective elastic modulus, from the volum fraction N phase fi, and elastic modulus N phase fraction M i as follow s [6] : The velocities of various types of seismic w aves in homogeneous, isotropic, elastic media are given by: Jizba [5] studied the effect of cementation on rock physics properties of sandstones, Avseth and M avko [2] show ed that the scatter observed in velocity-porosity data can be decomposed into depth-lines, w hile Dvorkin and Nur [4] show ed mathematically how cement could cause complexity in the velocity-porosity plane depending on cement location and mineral composition ( Figure 3). Rock texture and lithology also greatly affect the observed scatter (e.g., Bryant and Raikes [3]; Vernik [8]; Anselmetti and Eberli [1]).
Fi g 1. The study area w hich located in Java Sea know n as a carbonate rock reservoirs that produce gas.
Fi g 2. Simplification model to make the carbonate rock sample using effective medium theory.

M et hod
The reservoir param eters w ere obtained from a cross-section of 2D seismic data and the w elllogging borehole data. The w ell-logging data (Well -01) is show n by Figure 4.
The r s flow chart show n by Figure 5. Characteristic of reservoir rock could be know n from geological and seismic data input w hich could separated top and bottom of reservoir rock also its geometry. Log data w as used for making the crossplot w ith rock physics modelling. Fur ther this could separated zones w hich rich of shale and less of shale.
Fi gure 5. Flow chart of the research.

Result and Di scuss
2D seismic interpretation of study area show n by Figure 6. Carbonate reservoir characterized by the geometry that resemble a built -up carbonate and the flatspot of sesimic response. In addition, Figure 4 show n t he top (1000 ms) and the bottom (1060 ms) of reservoir that characterized w ith the changes of contrast acoustic impedance, density, and P-w ave velocity. Also there is an exsistence of a very long fault structure near the reservoir (left side).
Fi gure 6. 2D seismic interpretation in study area. From w ell-01 data and seismic response, w e could see that the geometry of carbonate reservoir show n by the existence of flatspot w hich resemble a built -up of carbonate rock.
Gamma ray crossplot and acoust ic impedance show n by Figure 7. The high gam ma ray show n the influence of shally-cemented in carbonate rock. Gamma ray data vs acoustic impedance separated zone of reservoir rock w hich rich of shale and less of shale.
Fi gure 7. Gamma ray crossplot vs acoustic impedance. The high gamma ray show n the influence of shally-cemented in carbonate rock.
Rock physics modeling in reservoir rock used the effective medium theory Voitgt, Reuss and Hashin-Shtrikman bounds. M odeling of composite reservoir rock (Figure 2) gave the result show n by curve in Figure 8. Elastic moduly (bulk moduly and shear moduly) decreasing proporsionally w ith increasing porosity value. The influence of shally-cement ed in carbonate rock causing the high bulk moduly. Figure 8c show n the reservoir rock zone w hich rich of shale and less of shale.
Fi gure 8. Effective medium theory using Voigt, Reuss, and Hashin-Shtrikman bounds. Figure 8a is bulk moduly, Figure 8b is shear moduly, and Figure 8c is prediction of shale-carbonate and initial carbonate zone.

P-w ave velocity
values is decreasing proporsionally w ith increasing porosity value. According to Figure 3, the effect of shallycemented in carbonat e gave a higher P-w ave velocity than the initial carbonate rock (Figure 9).
Fi gure 9. Effect shale-cemented in carbonate reservoir rock to P-w ave velocities value. Contact shale cemented in carbonate reservoir rock. Shally cemented on the carbonate rock obtain the P-w ave velocity higher than initial carbonate reservoir rock.

Concludi ng Rem ar ks
The conclusions from this paper are (1) Geometry and Top-Bottom reservoir carbonate rock can be know n from log data and seismic section, (2) The effect of shally-cement ed on reservoir rock characterized by an enhancm ent in bulk moduly and P-w ave velocity also a reduction in porosity, and (3) Rock physics modeling and log data could separated the characteristic of reservoir rock w hich shally-cement ed and its initial carbonate.