Formation Evaluation In The Geothermal Environment, The Geysers Steam Field, California
Authors: Cochran Larry E. 
Published: December 2007

The Geysers geothermal field produces steam from fractures occurring in graywacke rock within the Franciscan formation. This formation is a complex of deep oceanic crustal, sedimentary, and volcanic rocks, all of which are typically altered or metamorphosed.

A description of the complex rocks contained in the overlying section and reservoir of the Geysers field is required to optimize well design. Lithologic types, the location of potential steam productive zones, water flows and incompetent rocks are all factors critical to well design. A significant amount of this information is available from conventional wireline tools and mud-logging techniques.

The available evaluation data have been digitized for computer processing. These data include:conventional wireline log,sample descriptions,extrapolated temperature data, andsteam entry data.

Wireline log data include only the overlying or nonproductive reservoir sections. Sample descriptions are available through all sections and wells. Temperature data and steam entry data are available in numerous wells.

Relationships are derived empirically by computer processing all data on an equal basis. This "integrated" processing all data on an equal basis. This "integrated" approach more fully utilizes the available data, illustrating relationships that otherwise might remain unrecognized. The derived relationships include a lithology and porosity model using either density or acoustic data and neutron porosity. Relationships between the degree of metamorphism observed in graywacke samples and steam entry frequency and rate also are discussed.

Introduction

The Geysers geothermal field is located in the Mayacmas Mountains of the Northern Coastal Range Province of California, about 80 miles north of San Province of California, about 80 miles north of San Francisco (Fig. 1). The field produces steam from natural fractures occurring in graywacke rock within the Franciscan formation. The steam reservoir is vapor dominated and exists at average conditions of 460 degrees F and 480 psi.

This paper reviews data that Shell Oil Co. has acquired by drilling on two federal leases totaling 4,077 acres in The Geysers area. Since acquiring these leases in Jan. 1974, Shell has drilled 12 wells, not including redrills. Steam has been encountered in these wells as shallow as 3,800 ft and as deep as 8,500 ft, with rates as high as 220,000 lb/hour.

GEOLOGICAL FRAMEWORK

The Mayacmas Mountains are the core of a southeastward-plunging structure that is complicated by intense structural grain is evidenced by the topography. The Geysers field is located a few miles south of the Clear Lake volcanics (Cobb Mountain, Mt. Konocti) and a few miles northwest of the volcanic Mount Saint Helena. The Franciscan formation in The Geysers area, believed to represent a deep ocean trench or arc-trench gap deposit, is a thick assemblage of complex rock types. Details of The Geysers area geology can be found by referring to the excellent study of McLaughlin and Stanley. However, on Shell's leases, the Franciscan formation can be generalized by dividing into three main lithologic zones (Fig. 2).Upper zone consists of a serpentinized periodotite block. Nearly 3,000 ft thick on the periodotite block. Nearly 3,000 ft thick on the east part of Shell's leases but absent to the west.Intermediate zone consists of interbedded graywackes, greenstones and melanges. Massive greenstones, defining the base of the groundwater, are present on the west of Shell's leases but are thinner to the east.Lower zone consists of the graywacke reservoir zone.

The lithologic terms used here are also discussed by McLaughlin and Stanley.