Dolomitization of reservoir rocks in the Smackover Formation, southeastern Gulf Coast, U.S.A

Abstract

The dolomites of the Jurassic Smackover Formation have been the subject of considerable study in past decades, yet there is little agreement on the processes that produced these dolomites. Proposed mechanisms include: 1) reflux, 2) penecontemporaneous supratidal [sabkha], 3) seawater-meteoric mixing, 4) evaporative-reflux meteoric mixing, 5) confined aquifer mixing, 6) thermal convection, 7) seawater-seepage, and 8) evaporated seawater seepage. This study investigates dolomites of the Smackover Formation from Florida to Mississippi on the northern rim of the Gulf of Mexico. It assesses the applicability of these models by integrating all available work on the paragenetic sequence and dolomite textures and creating a regional synthesis of porosity and permeability, distribution of key diagenetic fabrics, stable oxygen, and carbon isotopes, strontium isotopes, fluid inclusions, and trace elements. The results show that reflux dolomite, modified by recrystallization, is the best explanation for this widely studied dolomite-bearing unit.The regional δ18O data are key in coming to this conclusion. There are two types of dolomites. δ18O-enriched dolomite occurs throughout the Smackover platform but is more abundant updip rather than the downdip part. In contrast, δ18O-depleted dolomites occur throughout the Smackover platform. We conclude that reflux dolomitization occurred during the early Oxfordian period as gypsum-saturated brines descended into the Smackover from the coeval deposition of Buckner evaporites, displacing connate seawater in the aquifer. As burial continued, salterns formed in the platform interior, as the Smackover shelf evolved from non-rimmed to rimmed. During this episode, gypsum-saturated brines descended deeply into the Smackover aquifer, further displacing the connate seawater remaining in the aquifer. This process continued until evaporated brine production ceased at the end of the Oxfordian when the carbonate/evaporate platform was flooded following transgression during the Kimmeridgian.Most importantly, a strong positive correlation between mean δ18O and burial depth suggests that recrystallization has produced as much as a 12‰ δ18O negative shift in the δ18O composition of these dolomites during 160 My of burial. In addition, δ18O-depleted saddle dolomite and calcite cement with more radiogenic 87Sr/86Sr ratios indicate dolomitization in water that had interacted with stratigraphically adjacent terrigenous-clastic units such as the Norphlet and Haynesville formations. Reflux was followed by prolonged thermally or compaction-driven fluid flow, hydrocarbon generation, and migration. Increasing temperature during burial catalyzed recrystallization and dolomitization well into the Late Cenozoic, producing the 12‰ δ18O spread.