Diagenetic and sequence stratigraphic controls on reservoir quality in the Upper Jurassic Smackover Formation, southern Arkansas, USA

Abstract

The Upper Jurassic Smackover Formation of the U.S. Gulf Coast Region is a mature hydrocarbon play that, in recent decades, has become a target for producing lithium-rich brines and a potential site for carbon storage. Despite this activity, the sequence stratigraphic framework of the Smackover Formation is widely disputed. One of the most challenging aspects of understanding the spatial and temporal controls on diagenesis is establishing the sequence stratigraphic framework. These uncertainties have resulted in uncertainty in delineating trends in porosity and permeability. Here, we characterize the reservoir quality, establish the sequence stratigraphic framework, and determine the spatio-temporal relationship between depositional and diagenetic processes in the Smackover Formation.

This study utilizes twelve subsurface cores from the Smackover Formation across southern Arkansas, including predominantly the Smackover “B” [Upper Jurassic (UJ) III Sequence] and, to a lesser degree, the Smackover “C” (UJ II Sequence). 215 well logs were correlated to assess the 3-D stratigraphic framework of the Smackover “B” and “C”. From a north-south oriented 2D seismic profile, the Smackover “B” is interpreted to consist of a series of progradational parasequences deposited over the lower-angle Smackover “C” ramp in southern Arkansas. Based on log-correlations, we interpret the Smackover “B” progradational parasequences to have top-set beds with a dip of 0.25° along the inner ramp, foreset beds with a dip of 0.5°–1.0° along the mid ramp, and bottomset beds with a dip of 0.25° along the outer ramp in this area. Based on these geometries, twenty-five parasequences were correlated within the Smackover “B”. The Smackover "B” and overlying Buckner Formation is characterized by seven different depositional facies interpreted to reflect a depositional profile that extends from coastal interdune salinas or sabkhas to a shallow marine mid-outer ramp setting. The Smackover “B” is interpreted to reflect a shallowing upward trend consisting of peloidal wacke-to-packstone overlain by ooid-oncoid grainstone, capped by silty dolomitic mudstone and shaley nodular anhydrite of the Buckner Formation. The underlying Smackover “C” consists predominantly of peloidal wacke-to-packstone. In general, the Smackover “B” and “C” have undergone varying degrees and styles of diagenesis, including cementation, dissolution, and dolomitization. Based on the classification of diagenetically altered depositional facies, sixteen diagenetic facies were identified. The Smackover “B” consists of alternating beds of thin-rim cemented ooid, tight fully cemented ooid, moldic ooid, and peloidal grainstones. Vertical patterns in the styles and degree of cementation and dissolution in the Smackover “B” is interpreted to reflect oscillations between the meteoric vadose, meteoric phreatic, and marine phreatic zones driven by low amplitude, high-frequency relative sea level changes within the UJ III sequence. Underlying the UJ III sequence boundary, the uppermost part of Smackover “C”/UJ II highstand is characterized by an increasing degree of dolomitization up section. The Smackover “C” dolomite are interpreted to have formed subsequently during the UJ III lowstand. This spatial and temporal variability in the degree and style of diagenesis in the Smackover Formation is interpreted to be driven by changes in relative sea level. This study demonstrates how the interplay between relative sea level drives the evolution of diagenetic fluids which in turn significantly impact reservoir quality in the Smackover Formation “B” and “C” in southern Arkansas.