Preservation of Porosity in the Deep Woodbine/Tuscaloosa Trend, Louisiana
Authors: Alan Thomson 
Published: December 2007

Cores of the Woodbine/Tuscaloosa formation from False River field, Pointe Coupee Parish, LA. contain sandstones from 20,000 ft (6562 m) with anomalously high porosities and permeabilities. Porosities greater than 25% and permeabilities of hundreds of millidarcies are common. Scanning electron microscopy (SEM) shows that individual grains of these olive-green semifriable sandstones are coated with chlorite. The chlorite occurs as 7 to 10 micron- (µm-) wide hexagonal plates that arrange themselves edgewise one crystal thick on grain surfaces. Sandstones with continuous chlorite coatings around quartz grains display little framework compaction and minor development of secondary quartz overgrowths; however, interbedded sandstones with little or no chlorite often are cemented completely by secondary quartz. Intermediate between these extremes are sandstones with incomplete or poorly developed chlorite coatings; these display outgrowths of secondary quartz rather than overgrowths of an envelope nature.

Petrographic and SEM data indicate an early diagenetic origin for the chlorite, which apparently ceased to form once detrital grains were coated with a single layer of crystals. This layer was sufficient to mask nucleation sites for silica overgrowths, and in addition may have prevented compaction by pressure solution, thereby allowing the sandstones to be buried to great depths without appreciably reducing porosity.

A late dissolution event resulted in the formation of some secondary porosity, but the bulk of the observed porosity is relict primary in origin, formed in the manner previously described.

Reservoir engineers and petrophysicists might ask themselves what effects the micropore system produced by the chlorite coatings will have on irreducible and critical water saturations as well as porosity logs.

Introduction

In mid-1965, Chevron Oil Co. discovered a deep gas accumulation in Pointe Coupee Parish, LA, in what is now called False River field. The field is 12 miles (20.81 km) downdip of the Lower Cretaceous Edwards shelf margin (Fig. 1) and production is from Woodbine/Tuscaloosa sandstones at about 20,000 ft (6462 m). A core of a water-bearing sandstone was taken from below 21,000 ft (6890 m) in the discovery well, Alma Plantation Well 1, with reported measured porosity values up to 26% (unstressed) and permeability values as much as 200 md. With this well the deep Woodbine/Tuscaloosa play was born.

That these porosity values are anomalously high is illustrated in Fig. 2, which plots porosity vs. depth for an average reservoir sandstone. The high porosity values for the Alma Plantation Well 1 depart markedly from the normal trend. This observation prompted our study.

Chlorite in a Deeply-Buried Sandstone

Petrography

Selected portions of the deep sandstone core in the Alma Plantation Well 1 are shown in Fig. 3, along with petrophysical data. Intervals with high porosity were recovered as crumbly fragments and are semifriable. The sandstones are fine grained and well sorted. The only important internal sedimentary structure is a faint graded bedding. A few marine shells are present, but adjacent shales are barren of fossils. Core samples from the nearby Crochet Well 1 also were examined. These samples ranged from crumbly and semi friable to hard and almost quartaitic. Core analyses of selected intervals from the two wells are shown in Tables 1 and 2.

Petrography

Selected portions of the deep sandstone core in the Alma Plantation Well 1 are shown in Fig. 3, along with petrophysical data. Intervals with high porosity were recovered as crumbly fragments and are semifriable. The sandstones are fine grained and well sorted. The only important internal sedimentary structure is a faint graded bedding. A few marine shells are present, but adjacent shales are barren of fossils. Core samples from the nearby Crochet Well 1 also were examined. These samples ranged from crumbly and semi friable to hard and almost quartaitic. Core analyses of selected intervals from the two wells are shown in Tables 1 and 2.