Modelling of Physical Petrophysical Characteristics of Potential Goru Sands in Lower Indus Basin, Pakistan

Abstract

Reservoir properties are essential for quantitative and qualitative reservoir evaluation. Although the interpretation of seismic data and petrophysical analysis of well logs provides a crude analysis of the reservoir, however, the spatial distribution of reservoir characteristics is essential for economical exploitation. Furthermore, the reservoir characterization based solely on seismic and petrophysical analysis is prone to ambiguities due to numerous factors that are generally not considered. To remove the errors encountered in the well log data, rock physics modelling is applied. The objective of this study is to characterize the reservoir properties of C interval in Gambat-Latif block, Lower Indus Basin, Pakistan using rock physics modeling for well log data of Tajjal 01, 02 and subsequent estimation of spatially distributed reservoir properties over a vintage 3D seismic data. C Interval has been identified using well to seismic tie, with horizons picked on the 3D seismic data. Interpreted seismic volume shows presence of three major faults in subsurface, with faults oriented in a NW-SE direction, showing marked horizons as shallow in west and deeper in the east, with deposition of facies also showing a thickening trend towards east. Petrophysical analysis conducted in both the wells, out of which two zones of interest were marked in Tajjal-01 well showing a higher average effective porosity ranging from 10%-12%, and a low water saturation ranging from 18.3%-19.6%, whereas in well Tajjal-02, the water saturation is high i.e., 87.4%. The template used to develop the rock physics model is the unconsolidated or poorly consolidated sand (using HRS utility). Based on this model, crossplots for Vp, Vs, density, acoustic impedance, and Vp/Vs ratio have been generated to differentiate sand and shale facies within the C interval, along with identification of the fluid in the sand lithology. Results of above mentioned crossplots showed the sand facies in Tajjal-01 well with a higher deflection away from trendline, indicating gas saturated sands, while no deflection away from trendline in these crossplots of Tajjal-02 well were observed, indicating sands in Tajjl-02 being water saturated. Furthermore, the derived quantitative properties from rock physics model have been utilized to spatially estimate the acoustic impedance, identify lithology, and confirm the presence of fluid in the C sands using model-based inversion and lambda-mu-rho attribute. The picked horizons have been used to mark the spatial distribution of the quantitative reservoir properties. Model based inversion depicts low acoustic impedance anomalous zones in the sands of C interval indicating the presence of hydrocarbons. The mu-rho attribute spatially indicates low to moderate values confirming the presence of sands, while the lambda-rho attribute, DRSML QAU 10 sensitive to pore fluids, confirms the presence of gas saturated sands of C interval. The results confirms that the spatially distributed properties estimated using refined well logs from rock physics model indicate the presence of hydrocarbons in the C-interval of Lower Goru Formation in the locality