Abstract

Fractured chert reservoirs of Devonian age in west Texas have produced over 700 million barrels of oil, and amount of mobile hydrocarbon remains intact. These reservoirs are characterized by microporosity, heterogeneity and compartmentalization, which result in low recovery rates (less than 30%). We cores, petrophysical logs, and 3-D seismic data to characterize the deep-water depositional facies and fracture distribution of these reservoirs on the Central Basin Platform. Gassmann’s fluid substitution method, which uses elastic properties measured from core samples, is validated to calculate the elastic properties and model the seismic responses to the variation of fluids in the reservoir rock. We utilize newly developed seismic attributes to infer distribution of various lithofacies, high-porosity zone and fractures. The relations between P-wave velocity and porosity measured from cores, between impedance and producing zone, and between initial production rates and seismic “fracture lineaments”.

Chert, as an unconventional reservoir rock, has been heavily developed in places such as West Texas, Oklahoma, California and Canada (Rogers and Longman, 2001). Our study is focused on the Devonian age chert deposition on the central basin platform, west Texas (Figure 1). This prolific reservoir rock still plays an important role after production of more than one half century.

For a reservoir, understanding the distribution of over a structure of interest is very critical to hydrocarbon exploration. Before 3-D seismic data was acquired, cores and well logs were used to correlate and interpolate the porosity distribution between wells and extrapolate the distribution beyond the area with well control. This method works well if the structure is sample and wells are densely populated over the area of interest. However, in many cases, there are no enough wells to control the area and the geology is more complicated than expected.

A 3-D seismic data volume of over 200 square km (14 by 15km) are available to us for research (Figure 1), The seismic data gave us much more details about the structure and stratigraphy (Figure 2 and Figure 3). More important, it also provides opportunities to create various seismic attributes to predict the rock properties and more detailed structural features in this reservoir rock.

With the above data, we attempt to define a detailed distribution of the porous chert and fluid content. We also attempt to characterize fractures in the chert reservoirs.