Quantifying Karst-filling Anhydrite Distributions from Pore to Log Scales; San Andres Formation, Permian Basin Part 1: Understanding the process

A. É. Csoma¹, R. M. Phelps¹, H. F. El-Sobky¹, J. Luczaj², R. H. Goldstein³, C. Liu¹, and X. Liu¹
¹ConocoPhillips, Houston, TX, USA
²University of Wisconsin – Green Bay, Green Bay, WI, USA
³University of Kansas, Lawrence, KS, USA

The San Andres Formation constitutes the most prolific production interval within the Permian Basin and is well-known to exhibit complex flow-unit heterogeneities that result from primary depositional facies, early meteoric karst over-printing, and from a multifaceted diagenetic history. Karst geobodies of the San Andres Formation can be open karstic pores that act as flow pathways (Yates field; Tinker et al., 1995) or anhydrite-filled karstic pores (MCA and Vacuum fields) that act as flow barriers. Understanding the origin of karst features and the diagenesis that leads to their filling is inevitable for their 3D prediction in the reservoir.

Two main karst horizons related to subaerial exposure in the Upper and Lower San Andres Formation have been studied at the MCA (Maljamar) and Vacuum fields, northwest shelf of the Delaware Basin in southeast New Mexico. The karst and associated fractures are in the studied fields are filled with anhydrite cement. The depth to which karstic pores penetrate below each of the two exposure surfaces is on average only 10 to 20 feet, and thus typically below seismic resolution. An alternative workflow utilizing the concept of quantitative diagenesis from pore-to-core-to-log-to-reservoir has been developed to determine the 3D distribution of anhydrite-filled karst at well locations.

This paper describes the diagenetic process responsible for the anhydrite cementation of the karst system as well as introduces the quantitative methodology from pore-to-log scales that are used to determine the 3D distribution of anhydrite-filled karst at well locations. For the detailed log analyses, please see Part II of this work by El-Sobky et al. at this conference.

Anhydrite is a common mineral of the San Andres Formation. Based on core observations, several morphologic classes of anhydrite bodies are identified, including (1) displacive, (2) fenestrae-filling, (3) skeletal mold-filling, (4) fracture-filling, (5) breccia-filling, and (6) massive. Based on cross-cutting relationships and superposition, anhydrite classes 1 and 2 are interpreted to be of depositional or early diagenetic in origin and class 3 is likely related to intermediate stages of diagenesis. The origin of anhydrite classes 4-6 were further investigated by thin section and fluid inclusion analyses. Fracture-, breccia-filling and massive anhydrites post-date coarse-crystalline dolomite cement and are in association with other minerals such as late-stage pyrite, kaolinite, and quartz. On the basis of fluid inclusion Th and Tm ice data, dolomite cement has formed from >~23 wt.% NaCl equivalent dense brines at 62 to 85°C. Coarsely crystalline anhydrite postdated dolomite cement and formed from lower salinity fluids than those of the dolomite cement (12 to 16 wt.% NaCl equivalent) at minimum of 85 to 95°C. Based on the basin model, late stage dolomite likely formed from burial fluids at maximum burial whereas coarsely crystalline anhydrite formed during elevated heat-flow from mixed recharging regional freshwater and hydrothermal fluids during Basin and Range deformation in the Tertiary.

Understanding the origin of late-stage karst-filling dolomites is critical to the quantitative evaluation of anhydrite-filled karst, in which an early subaerial event (karst formation) is coupled with late collapse and anhydrite cementation forming karst geobodies. In this study, volumetrics of anhydrites were used to map out the extent of karst features.

Image analysis (Core Image Segmentation) on box-photos from two San Andres Formation cores was performed with a free image analysis software package, JMicroVision. Vertical proportions of anhydrite were quantified at the millimeter-scale, assigned an anhydrite class, and then up-scaled within 4-inch windows for numerical analysis with well-log suites.

The results indicate that late-stage anhydrite classes are associated with karst breccias and their related fracture networks are volumetrically most abundant and occur in at least 2 stratigraphic intervals beneath regional subaerial exposure surfaces. Based on calibration of the core image analyses, when more than 30% anhydrite is present in a given interval, it is related to karst and fracture-filling anhydrite of anhydrite classes 4-6.

Anhydrite-filled karst breccias are expected to form spatially complex geobodies that baffle fluid flow and may significantly affect the efficiency of tertiary recovery methods in the San Andres interval. This concept served as the impetus for an anhydrite-mapping plan, where a deterministic and statistical technique was used to test the sensitivity of the wireline logging response to the core-derived anhydrite volume. Successful anhydrite core/log correlation is used to build a forward neural network model that facilitates anhydrite volumetric estimation and distribution. Results from the neural network model will be input into a three-dimensional geocellular model, in which the anhydrite-filled karst networks are realized by using multipoint statistical technique with customized training images. The final model provides a method for identifying flow pathways within the reservoir and for highlighting potential missed pay zones isolated by the anhydrite barriers."

This type of quantitative analysis may be applied in additional ConocoPhillips assets with a significant anhydrite presence, while further work may establish similar work flows for three-dimensional mapping of reservoir quality properties such as vuggy porosity and thin heterolithic beds.

AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012