Determining the Type of Fluid Contained in the Fractures of the Twin Creek Limestone By Using the Dual Laterolog (TM) Tool
Authors: Rasmus John C. 
Published: December 2007

Summary
This paper describes an experimental log evaluation technique to identify and to evaluate the fractured intervals occurring within the Twin Creek limestone of the Overthrust belt. The area of study is the Yellow Creek field located approximately 3 miles southwest of Evanston, WY. The technique uses the response characteristics of the two resistivity curves from the Dual Later log (TM) (DLL) service to locate the fractured intervals and to identify the fluid type within the fractures. Example logs and cross plots are shown.

Introduction
Several log measurements and techniques are being used to locate natural fracture systems occurring in various producing reservoir's. This paper describes an empirical technique using the DLL service to identify fractured intervals and to predict the production type.
In a reservoir with very low porosity, the resistivity from an electrical device such as a laterolog tool can be affected greatly by a fracture system filled with conductive fluids. A device with a shallow depth of investigation is more affected than one that reads more deeply. This paper develops a theory for this phenomenon and describes application and interpretation of the DLL technique in the Yellow Creek field.

Reservoir Characteristics
The Twin Creek limestone in the Overthrust belt was deposited during Jurassic time and lies above the Nugget sandstone and below the Preuss sandstone (Fig. 1). It is composed of seven members, each with various degrees of shaliness. The uppermost is the Giraffe Creek member, lithologically characterized as silty to finely sandy, thin-bedded limestone. Below this lies the Leads Creek member, consisting of a soft, shaly limestone that grades into the cleaner, denser Watton Canyon member. The basal bed of the Watton Canyon is a massive, dense, brittle, thin-bedded limestone with 0 to 3% primary porosity. Where the Watton Canyon member outcrops, it weathers into a noticeable rectangular fracture pattern. Tectonic forces acting on this member probably have produced similar fracture patterns. The Watton Canyon grades into the shaly siltstone of the Boundary Ridge member. Directly below this is the Rich member, characterized as a shaly, splintery limestone that becomes softer and shalier at the base, where it forms a sharp boundary with the hard, thin-bedded oolitic limestone of the Slide rock member. Below the Slide rock lies the Gypsum Springs member, consisting of anhydrite and brecciated limestone beds that overlie the Nugget sandstone.
The Watton Canyon member is the most prolific producer in the Twin Creek limestone, although hydrocarbons also have been produced from the Leads Creek, Rich, Slide rock, and Gypsum Springs members.
Production from the limestone consists of gas with various amounts of condensate and oil. Initial production rates from the Watton Canyon member have been as high as 12 MMcf/D with condensate. An independent study of cores from the Watton Canyon member has found that the total porosity averages I %, with fractures and matrix each contributing 0.5%. Reservoir behavior indicates that the matrix porosity is water saturated, with only the fracture system contributing to the storage and production of hydrocarbons.

Log Response
Porosities from the sonic and neutron-density cross plot range between 0 and 3 % in the Watton Canyon member. Both track core porosities closely. There is no real indication of secondary porosity from the log measurements at these low porosities.
JPT
P. 2673^