Ichnology and Paleopedology: Keys to Understanding Reservoir Quality in Continental-Estuarine Deposits of the Donovan Sand (Lower Cretaceous), Citronelle Field, Alabama
Pashin, Jack; Kopaska-Merkel, David C.; Arnold, Ann C.
Biological and pedogenic processes have a strong effect on reservoir quality and heterogeneity, yet are not routinely evaluated when characterizing reservoirs. Citronelle Field is a major Gulf Coast oil reservoir that has produced 170 MMbbl of 42-46° API gravity oil from the Donovan Sand. Original oil-in-place is estimated to be 537 MMbbl, and CO2-enhanced oil recovery may increase incremental recovery by 80 MMbbl. Detailed lithological, petrological, and petrophysical analysis of cores and well logs is being used to characterize reservoir architecture and to assist in reservoir management.
Producing from 43 distinct sandstone bodies, the Donovan Sand spans more than 900 feet of section. Sandstone bodies are arranged in two retrogradational sequence sets, each dominated by continental deposits with estuarine deposits near the top. Individual depositional sequences contain an aggradational succession of conglomerate and sandstone that grades upward into red and gray mudstone. The conglomerate and sandstone represent incised valley fills, and the mudstone units are mainly vertic paleosols and, less commonly, estuarine bayfill and mudflat deposits. Continental deposits are dominated by meniscate burrows and contain other ichnofossils resembling Steinichnus and Camborygma. Estuarine strata, by comparison, include a diverse fossil assemblage that includes Teichichnus, Rhizocorrallium, oysters, and foraminifera.
Pedogenesis and bioturbation are critical determinants of porosity and permeability in the Donovan Sand. Feldspar dissolution by meteoric flushing accounts for >40% of the porosity in the pay column. Conversely, accumulation of illuvial clay mobilized by flushing occludes porosity such that pay geometry and sandstone geometry do not correspond. Bioturbation is widespread in the upper parts of the sandstone bodies and the overlying mudstone units and can increase or decrease permeability by an order of magnitude. Ichnologic processes affecting permeability include pelletization, alteration of grain packing, and mechanical mixing of mud and sand. Bioturbation further created pathways for dissolution of feldspar and the accumulation of authigenic minerals and pyrobitumen. Results of this study illustrate the predictive value of ichnology and paleopedology as tools for characterizing mature reservoirs and refining strategies for tertiary oil recovery.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013