Petrophysical Evaluation of the Diatomite Formation of the Lost Hills Field, California
Authors: Stosur J.J. , David A. 
Published: December 2007

Renewed interest in the vast oil reserves of the Lost Hills field led to the evaluation of the petrophysical properties obtained from logs and cores. A method known as pattern recognition was used to gain insight into the properties of the diatomite beds, which are unlike those of most other formations.

Introduction

The Lost Hills oil field in California contains several producing horizons. The Etchegoin formation of the producing horizons. The Etchegoin formation of the Pliocene series consists of soft, massive diatomite beds of Pliocene series consists of soft, massive diatomite beds of very high porosity and exceptionally low permeability. In many ways, the matrix resembles a chalk formation. The depth of the Main Zone ranges from about 900 to 1,900 ft. The wells are usually completed with slotted liners across 600 to 1,000 ft, of which the net productive thickness is about 500 ft.

All the more permeable beds within the Main Zone appear to be depleted and show signs of gas saturation, particularly in the structurally higher locations. Reservoir particularly in the structurally higher locations. Reservoir pressure varies from about 400 psi in tight beds to nearly pressure varies from about 400 psi in tight beds to nearly atmospheric in the more permeable beds. Oil is contained in diatomaceous earth that is mixed with very fine sand and clay at variable ratios.

Most of the Main Zone has a permeability on the order of only a few millidarcies, despite a reservoir porosity that frequently exceeds 40 percent. There seems to be a relationship between the purity of the diatomite and its porosity and permeability - the cleaner the diatomite, porosity and permeability - the cleaner the diatomite, the higher the porosity and the lower the permeability.

A number of wells were recently drilled in a test area in Lost Hills. The suite of logs run in five of the wells included the dual-induction laterolog with SP and the density-compensated neutron log combined with gamma ray and caliper. A type log is shown in Fig. 1. One well was cored through a zone of interest. The objective of the logging program was fluid-saturation determination, with emphasis on the detection of possible gas saturation that was considered deleterious to the outcome of a pilot test. The information collected from these and a few other wells and cores obtained several years earlier permitted some observations about the unusual nature of this reservoir. Standard laboratory core analysis with petrographic, X-ray, optical microscope, and scanning petrographic, X-ray, optical microscope, and scanning electron microscope examinations was compared with results obtained from log analysis using the "pattern recognition" approach.

Diatomaceous Earth and Its Properties

A diatomaceous deposit is a hydrous, noncrystalline form of silica or opal composed of microscopic shells of diatoms, which are microscopic, single-celled, aquatic plankton plants. It is light-colored and resembles chalk, plankton plants. It is light-colored and resembles chalk, but is much less dense and will not effervesce in acid as chalk does. The diatom forms can be distinguished under a high-power microscope. Thousands of varieties are known. Their skeletons accumulate at the bottom of marine environments. The empty shells are practically imperishable, and they accumulate to form very thick beds of "diatomite ooze." Ref. 4 gives a good description of the structure and the incredible variety of forms of the diatoms.

It appears that, as small as the diatoms are, they contain some internal porosity detectable by a porosity device such as a density, sonic, or neutron log.

JPT

P. 1138