Ankerite

About: Ankerite is a research topic. Over the lifetime, 859 publications have been published within this topic receiving 23960 citations.

Depositional Influences in Sandstone Diagenesis of Lower Cretaceous Hosston Formation, Marion and Walthall Counties, Mississippi: ABSTRACT

Abstract: Poor reservoir quality is a significant impediment to exploration success in the deep Hosston gas/condensate trend of south-central Mississippi. Regionally, depositional lithology of the Hosston exerts significant influence on cementation and porosity reduction. The primary variables are depositional carbonate content, quartz sand-grain size, and the presence of matrix clay. Feldspar content is a variable of lesser significance. In Hosston fluvial and deltaic facies, carbonate content is low, and coarser sediments are present in channel sandstones. These coarser sandstones preserve commercial porosities, whereas finer-grained and shaley sandstones offer marginal porosities. Quartz overgrowths, pressure solution, and authigenic kaolinite are responsible for most diagenetic porosity reduction in this facies. In Hosston marine, prodeltaic sandstones, depositional carbonate is more common. Carbonate cements, dominantly ankerite and dolomite, are recrystallized from shell fragments and carbonate grains to occlude porosity. Quartz overgrowths, pressure solution, and stylolitization are also common in these finer sandstones. In distal, marine shelf sandstones, depositional carbonate is abundant, and quartz sand is very fine-grained. Porosities are very low in this lithofacies because of pervasive early carbonate cementation and later quartz overgrowths in intervals not cemented by carbonate. The first stage of Hosston sandstone diagenesis was early calcite or quartz cementation. Calcite was later replaced by ankerite or dolomite. Deeper burial initiated plagioclase feldspar dissolution and the development of pore-filling kaolinite. Hydrocarbons subsequently accumulated in structural and stratigraphic closures and retarded further diagenetic porosity reduction in these sandstones. Deeper burial caused thermal degradation of oil reservoirs to dry gas and pyrobitumen. Minor galena, sphalerite, pyrite, and barite are present in open, tensional fractures, associated with stylolites and as intergranular cements. These sulfides and sulfates were probably precipitated from metalliferous brines common in the Hosston.

Geological and geochemical characteristis of the Changba and Dengjiashan Pb-Zn deposits in the Qinling orogenic belt, China

Abstract: The Changba and Dengjiashan deposits are characterized by stratiform and lenticular orebodies with bedded and laminated structures. The major minerals are sphalerite, pyrite, galena, quartz, calcite, ankerite and barite. They are rich in 34S and Eu, depleted in Ce, and show a Pb isotope signature between the Upper Crust and Mantle reservoirs. The world-class Changba deposit is hosted in Mid-Devonian Eifelian clastic rocks and limestone, underlain by a lens of sulfide-cemented breccia. The Dengjiashan deposit, however, is hosted in younger Givetian chert, with more breccia and vein type ores. It is concluded that the deposits formed as exhalative accumulationsi in Mid-Devonian sedimentary basins. The Changba deposit formed on a continental slope, whereas the Dengjiashan deposit formed in an organic bank environment.

Carbonatite-Alkaline Complex of Mundwara

Abstract: Two types of carbonatite dykes - alvikite and beforsite are associated with (the Mundwara alkaline igneous complex. Petrochemical and mineralogical studies indicate enrichment of alkalies and niobium. Pyrochlore is noted in beforsites along with dolomite, apatite, barite and flourite, whereas calcite, ankerite, heamatite, aegirine, apatite and barite are noted in the alvikites. The presence of both metasomatic as well as intrusive syenites are noticed. Higher content of Nb is noted in the syenites containing sphene, aegirine and magnetite. The presence of annular fractures around the plutons and the arcuate shatter zone with breccia on the outer margins of the Mer pluton indicate upwarping and later intrusion of carbonatites through the Erinpura granite. High P 2 O 5 and Nb content of the soil samples around Mer Pluton suggest possible existence of a carbonatite plug below.

Sedimentological, mineralogical and geochemical definition of oil-shale facies in the lower Parachute Creek Member of Green River Formation, Colorado

Abstract: Sedimentological, mineralogical and geochemical studies of two drill cores penetrating the lower Saline zone of the Parachute Creek Member (middle L-4 oil-shale zone through upper R-2 zone) of the Green River Formation in north-central Piceance Creek basin, Colorado, indicate the presence of two distinct oil-shale facies. The most abundant facies has laminated stratification and frequently occurs in the L-4, L-3 and L-2 oil-shale zones. The second, and subordinate facies, has ''streaked and blebby'' stratification and is most abundant in the R-4, R-3 and R-2 zones. Laminated oil shale originated by slow, regular sedimentation during meromictic phases of ancient Lake Uinta, whereas streaked and blebby oil shale was deposited by episodic, non-channelized turbidity currents. Laminated oil shale has higher contents of nahcolite, dawsonite, quartz, K-feldspar and calcite, but less dolomite/ankerite and albite than streaked and blebby oil shale. Ca-Mg-Fe carbonate minerals in laminated oil shale have more variable compositions than those in streaked and blebby shales. Streaked and blebby oil shale has more kerogen and a greater diversity of kerogen particles than laminated oil shale. Such variations may produce different pyrolysis reactions when each shale type is retorted.