Sequence Stratigraphy, Sedimentology, and Economic Importance of Evaporites and Evaporite-Carbonate Transitions
J. F. "Rick" Sarg
ExxonMobil Exploration, Houston, TX
World-class hydrocarbon accumulations occur in many ancient evaporite-related basins. Seals and traps of such accumulations are, in many cases, controlled by the stratigraphic distribution of carbonate-evaporite facies transitions. Evaporites may occur in each of the systems tracts within depositional sequences. Condensed sections of organic-rich black lime mudstones that occur in basinal areas seaward of associated transgressive and highstand carbonate platforms have high preservation potential in evaporitic basins can be significant sources of hydrocarbons. Thick evaporite successions are best developed during sea level lowstands due to evaporative drawdown. Type 1 lowstand evaporite systems are characterized by thick wedges that fill basin centers, and onlap basin margins. Very thick successions (i.e., saline giants) represent 2nd-order supersequence set (2050 m.y.) lowstand systems that cap basin fills, and provide the ultimate top seals for the hydrocarbons contained within such basins.
Large-scale marine evaporites have been deposited in the Phanerozoic when tectonic-eustatic-climatic conditions have combined to provide basin restriction and net evaporative conditions. These halite-dominated saline giants generally occur in low-latitude regions at distinct times in earth history that are characterized by widespread withdrawal of marine waters from continental shelves (i.e., 2nd-order sea level lowstands), aridity, and where basin architecture and the surrounding landmasses provided restriction of marine waters. The saline giants have occurred under both greenhouse and icehouse conditions. They are not sudden events, but are often preceded by cyclic carbonate-evaporite sequences that reflect progressive climatic deterioration and basin restriction. Conditions conducive to large-scale evaporite deposition are not present in the Holocene. Sea level is at highstand, global oceans are well circulated, continental dispersion is at a maximum, and there is little potential for basin restriction in low latitudes.
Saline giants can be grouped into three tectonic settings that are prone to hydrographic restriction: continental interior sag basins; subbasins within post-orogenic distal forelands; and late-stage synrift basins partially connected to oceanic regimes. The Lower Paleozoic giants occur in continental interior sag basins, and were deposited in the EarlyMiddle Cambrian and Middle Ordovician (Siberia), the Late Silurian (Michigan basin), and the Middle Devonian (western Canada). The Late Paleozoic and Late Neogene giants occur in post-orogenic distal foreland basins. These include the Middle Pennsylvanian Paradox salts of the Four Corners region, western United States, the Middle Permian of the Pricaspian, and the Late Permian along a trans-highland region that stretched from west Texas through northern Europe and Arabia to central Russia. Thick salt deposits occurred during the Mesozoic (Lower-Middle Triassic, Middle Jurassic, and Aptian) in late-stage synrift Atlantic basins.
Evaporite-carbonate transitions at the 3rd-order and higher scale occur in each of the systems tracts and can result in hydrocarbon seal and trap potential. Where reservoir-bearing slope carbonate buildups occur, lowstand evaporites that onlap and overlap these buildups show a lateral facies mosaic directly related to the paleo-relief of the buildups. This facies mosaic, as exemplified in the Silurian of the Michigan basin, ranges from nodular mosaic anhydrite of supratidal sabkha origin deposited over the crests of the buildups, to downslope subaqueous facies of bedded massive/mosaic anhydrite and allochthonous dolomite-anhydrite breccias. Facies transitions near the updip onlap edges of evaporite wedges can provide lateral seals to hydrocarbons. Porous dolomites at the updip edges of lowstand evaporites will trap hydrocarbons where they onlap nonporous platform slope deposits. The Desert Creek Member of the Paradox Formation illustrates this transition. On the margins of the giant Aneth oil field in southeastern Utah, separate downdip oil pools have accumulated where dolomudstones and dolowackestones with microcrystalline porosity onlap the underlying highstand platform slope.
Where lowstand carbonate units exist in arid basins, the updip facies change from carbonates to evaporite-rich facies can also provide traps for hydrocarbons. The change from porous dolomites composed of high-energy, shallow-water grainstones and packstones to nonporous evaporitic lagoonal dolomite and sabkha anhydrite occurs in the Upper Permian San Andres/Grayburg sequences of the Permian basin. This facies change provides the trap for secondary oil pools on the basinward flanks of fields that are productive from highstand facies identical to the lowstand dolograinstones. Type 2 lowstand systems, like the Smackover Limestone of the Gulf of Mexico, show a similar relationship. Commonly, these evaporite systems are a facies mosaic of salina and sabkha evaporites admixed with wadi siliciclastics. They overlie and seal highstand carbonate platforms containing reservoir facies of shoalwater nonskeletal and skeletal grainstones. Further basinward these evaporites change facies into similar porous platform facies, and contain separate hydrocarbon traps.
Transgressions in arid settings over underfilled platforms (e.g., Zechstein (Permian) of Europe; Ferry Lake Anhydrite (Cretaceous), Gulf of Mexico) can result in deposition of alternating cyclic carbonates and evaporites in broad, shallow subaqueous hypersaline environments. Evaporites include bedded and palmate gypsum layers. Mudstones and wackestones are deposited in mesosaline, shallow subtidal to low intertidal environments during periodic flooding of the platform interior.
Highstand systems tracts are characterized by thick successions of m-scale, brining upward parasequences in platform interior settings. The Seven Rivers Formation (Guadalupian) of the Permian basin typifies this transition. An intertonguing of carbonate and sulfates is interpreted to occur in a broad, shallow subaqueous hypersaline shelf lagoon behind the main restricting shelf-edge carbonate complex. Underlying paleodepositional highs appear to control the position of the initial facies transition. Periodic flooding of the shelf interior results in widespread carbonate deposition comprised of mesosaline, skeletal-poor peloid dolowackestones/mudstones. Progressive restriction due to active carbonate deposition and/or an environment of net evaporation causes brining upward and deposition of lagoonal gypsum.
AAPG Search and Discovery Article #90015©2002-2003 AAPG Distinguished Lectures