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Fracture Development in Salt Dome Caprock, Hardin County, Texas*

By

Alfred Lacazette1, Andrew R. Thomas2, and Dennis Kuhfal3

 

Search and Discovery Article #50058 (2007)

Posted November 20, 2007

 

*Adapted from extended abstract prepared for presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007

 

1NaturalFractures.com LLC, Houston, TX ([email protected])  

2Chevron Energy Technology Company, Houston, TX

3Chevron Mid-Continent Business Unit, Houston, Texas 

 

Introduction 

The first East Texas oil boom was based on production from fractured limestone saltdome caprock. Limestone caprocks are formed during salt dome ascent by:

·         Accumulation of disseminated anhydrite due to salt dissolution,

·         Conversion of anhydrite and hydrocarbon to calcite and sulfides by sulfate reducing bacteria.

Fracture development in salt-dome caprock is understood poorly because caprock production was exhausted long before the advent of modern reservoir characterization technologies. This contribution describes a detailed study of the fracture system of the Sour Lake Field caprock (discovered in 1902) using oriented core, borehole image logs, crosswell seismic tomography, dipole sonic logs, petrography, isotopes, and other modern methods.

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Figure Captions

Figure 1. Location of Sour Lake Field (from Halbouty and Hardin, 1955) and contour map on top of salt (from Looff, 2001).

Figure 2. Caprock layers in thin-section at sample depth of 1737 ft. in the 816 well. The dark bands contain bitumen and very fine-grained calcite crystals. The bitumen is composed of heavy hydrocarbon that the bacteria could not consume. The cyclic layering of bitumen and coarse calcite is interpreted to represent cycles of bacterial growth and quiescence in the caprock accretion cycle. Thin-section photomicrograph, plane-polarized light; scale bar at lower right is 0.8mm.

Figure 3. Carbon and oxygen isotopic composition, Sour Lake limestone and anhydrite fracture zones. Plot shows a mixing line that we interpret to result from mixing of meteoric water and formation water influenced by sulfate-reducing bacteria.

Figure 4. Paleostress analysis of faults measured in core and the image log show vertical shortening and horizontal extension of the caprock, indicating that the caprock was stretched by dome growth. The intermediate axis is roughly parallel to the regional maximum compressive stress orientation suggesting that the regional stress state overprints the local stresses caused by dome emplacement. This stretching also produces extensional fractures (not shown).

Figure 5. Diagenetic shrinkage at a minor fault in anhydrite.

Left - The surface of a minor fault in the anhydrite layer shows polygonal extension cracks that are widest at the fault surface.

Right - Photomicrograph of thin-section cut at right angles to both the fault and one of the extension cracks. Calcite is stained pink; anhydrite is white; epoxy is blue. The fault surface (left side) is completely altered to calcite. Both calcite alteration and the crack die-out away from the fault surface. Note residual bitumen layer parallel to the fault surface, resulting from oil movement along the fault. Bacteria consume the hydrocarbon, convert sulfate to sulfide, generate bicarbonate, and back precipitate calcite in the presence of calcium. Conversion of anhydrite to calcite results in a 20% molar volume reduction. This volume change shrinks the caprock and produces extensional fractures that contribute to the fracture system.

Figure 6. FMI image of a pod of porous, fractured anhydrite in the 816 well between 1800 and 1900 feet.

Figure 7. Geologic interpretation of the crosswell velocity and diffraction tomograms between the 795 and 816 wells, based on well data (core, image logs, crossed-dipole sonic logs, and conventional logs). The yellow circle marks where the horizontal well penetrated the plane of the tomogram. Prominent faults are shown as heavy dashed lines with their apparent dip angles as measured in the plane of the tomogram. Vertical:horizontal scale = 1:1.

 

Data Set 

The following data was collected:

·         A vertical pilot hole (the 816) was drilled and continuously cored from the overlying shale, through the caprock, and into the underlying halite. A full suite of logs was collected in the 816, including an electrical image log and a crossed-dipole sonic log.

·         A crosswell tomogram was shot between the 816 and an existing well (the 795) that had a suite of conventional logs.

·         A horizontal well was drilled through the plane of the tomogram, and was oriented to optimally sample the fracture network based on observations in the pilot hole.

·         The horizontal well was logged and an image log was collected.

 

Results 

We find that the caprock fracture system developed during caprock growth and that development of the fracture system is linked to salt-dome movement. The caprock was fractured both by faulting and extensional fracturing resulting from dome emplacement and by mineral volume changes occurring during conversion of anhydrite to calcite. Isotopic data from the limestones (d18O: -4 to -8 PDB, and d13C: -11 to -31 PDB) suggest that fluids evolved from deep bacterial to mixed meteoric during dome ascent. Paleostress analysis based on fault-slip kinematics and extensional fracturing shows that caprock strain was due to dome inflation and that this expansion bears an overprint of the neotectonic (present-day) stress field. We interpret that previously unrecognized pockets of fractured limestone exist deep within the Sour Lake anhydrite layer and that these pockets represent a significant positive modification of traditional salt dome caprock reservoir volume. Some of these pockets are both sampled by wells and imaged on the crosswell tomogram. The fractured limestone pockets within the anhydrite resulted from bacterially-mediated diagenetic reactions that occurred when petroleum and brine flowed through faults resulting from dome expansion.  

This study targeted an undrilled area that was thought to represent an isolated fault block. The caprock was found to have a very well developed fracture system and to have excellent fracture porosity and permeability. Although the core from the pilot hole was oil soaked and although there were numerous oil shows during drilling of the horizontal well, the horizontal well tested ~30,000 barrels of water/day and little oil. We speculate that the fault block is not isolated and that the oil was produced and/or flushed-out by disposal water that was injected elsewhere in the dome.

 

Application 

The conditions that produced the fracture porosity and permeability at Sour Lake are the same processes that operate on all salt domes that develop caprock. Consequently, Sour Lake could be a typical example of a caprock fracture system and results of this study might be applied more broadly to caprocks in mobile salt basins. This study has direct application to oil and gas operations and construction of storage facilities in salt domes. The discovery of porous volumes within the anhydrite layer is especially significant because until now only the limestone layer was thought to have porosity and permeability.

 

References 

Halbouty, M.T., and G.C. Hardin, Jr., 1955, Factors affecting quantity of oil accumulation around some Texas Gulf Coast piercement-type salt domes: AAPG Bulletin, v. 39, p. 697-711.

Looff, K., M., 2001, Recent salt related uplift and subsidence at Sour Lake salt dome, Hardin County, Texas: GCAGS Transactions, v. 51, p. 187-194.

 

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