M. L. Ramadhan  and S. Winardhi 
 Geophysical Engineering Master Program, Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung, Ganesha 10, Bandung, 40132, Indonesia
 Reservoir Geophysics Research Group, Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung, Ganesha 10, Bandung, 40132, Indonesia
Corresponding author: email@example.com
Oil and gas production in Indonesia has significantly declined in recent years due to the decrease of conventional reserves. Therefore oil and gas industry needs to start the exploration and development of unconventional reservoir, such as shale oil or gas, that is found trapped within its original source rock, the organic-rich shale. Organic shale may contain kerogen as organic matter that evolved to hydrocarbon along with maturity process. It requires deep knowledge of rock physics relation between reservoir properties and elastic parameters in shales affected by kerogen. We attempt to quantify the effect of kerogen content and its porosity on elastic and geo-mechanical properties, as well as seismic responses based on rock physics modeling. Results show that with the increase of kerogen content and porosity in organic shales, the P-wave and S-wave velocity as well as density generally decrease, Young Modulus also decreases, while Poisson’s ratio increases only with kerogen content. For AVO forward modeling, AVO response shows class IV response indicated by negative intercept and positive gradient.
The decrease of conventional reservoir as the main source of hydrocarbon is one of the problems. Therefore oil and gas industry needs to start exploring and developing unconventional reservoir, such as shale oil and shale gas. In this situation, producing shale oil or gas appears to be the solution to replace conventional oil and gas. Moreover it is estimated that Indonesia has abundant shale oil and shale gas resources, reaching 574 TCF, that can potentially fulfill energy demand in the future .
The study of effective elastic properties based on rock physics modeling can help improving our understanding of this unconventional resource. Several studies on the elastic properties of shale have been conducted. Zhu et al.  developed an improved rock physics workflow to incorporate TOC effects; Li et al.  used rock physics modeling to characterize organic-rich shale. These two models are combined in this study to simulate the elastic properties of organic-rich shales. Attempt was made in quantifying the effects of two important factors, kerogen content and kerogen porosity, on elastic properties as well as geo-mechanical and seismic responses.
Rock-physics models can provide crucial links between microscopic rock properties and macroscopic physical characteristics as the basis for predicting rock and fluid properties from geophysical data . The constructed rock physics model is based on isotropic assumption and focuses on quantifying the effect of kerogen on elastic properties of shales. To account for the complexity of organic-rich shale with inclusion-based microporosity theory, we propose two models as shown in figure 1.
The first model is the extension of incorporated organic matter as a rock constituent initially proposed by Zhu, called as modified Zhu model. The second model is the modified kerogen microporosity model previously developed by Li, called as modified Li model. The two models are basically identical, but there is a slight difference in calcucation method where Gassmann theory to infill hydrocarbon into kerogen pores in modified Zhu (number 1 in figure 1) is replaced by direct inclusion-fill of Kuster-Toksoz in modified Li (number 2 in figure 1).
Table 1. Constituent material properties.
|Material||Fraction||Density, g/cm3||Bulk modulus, GPa||Shear modulus, GPa|
|Quartz||0.3 – 0.4||2.65||38||44|
|Kerogen (K)||0 – 0.1||1.4||6.78||2.02|
|Gas (ϕk)||(0 – 0.5)K||0.11||0.04||0|