Petrologic characteristics and genesis of dolostone from the Campanian of the SK-I Well Core in the Songliao Basin, China

Abstract

The well SK-I in the Songliao Basin is the first scientific borehole targeting the continental Cretaceous strata in China. Oval concretions, thin laminae and beds of dolostone are found intercalated within mudstone and organic-rich black shale in the Nenjiang Formation of Campanian age. Low ordered ferruginous dolomite is composed of euhedralesubhedral rhombs with cloudy nucleus and light rims formed during the diagenesis, which are typical features of replacement. The heavy carbon isotopes (d 13 C PDB e 1.16e16.0) are results of both the fermentation of organic matter by microbes and degassing of carbon dioxide during the period of diagenesis, and the presence of light oxygen isotopes (d 18 O PDB e 18.53wÀ5.1) is a characteristic feature of fresh water influence which means the carbonate may have been altered by ground water or rainwater in the late diagenesis. Marine water incursions into the normally lacustrine basin have been proved by both the salinity of Z value and the occurrence of foraminifera in the same strata where dolomite occurs. Pyrite framboids observed by SEM are usually enclosed in the dolomite crystals or in the mudstones, supporting the sulfate reducing bacteria (SRB). The formation of both dolomite and pyrite are associated with marine water incursions, which not only supply magnesium GEOSCIENCE FRONTIERS journal homepage: www.elsevier.com/locate/gsf GEOSCIENCE FRONTIERS 3(5) (2012) 669e680 ion for dolomite, but also result in limited carbonate precipitation in the basin. The presence of pyrite framboids indicates the development of an anoxic environment associated with salinity stratification in the lake. The dolomite in the Nenjiang Formation is the results of marine water incursions, diagenetic replacement of calcareous carbonate and sulfate reducing bacteria (SRB). ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

Experimental investigation, binary modelling and artificial neural network prediction of surfactant adsorption for enhanced oil recovery application

“…Three main peaks were observed at 20.84◦, 26.63◦ and 50.12◦, then they were compared to JCPDS record which indicated the high quartz presence in the sample. The other minor peaks indicated the low quantity presence of quartz, kaolinite, illite and Illite-Smectite [67] , [68] . A semi-quantitative mineral composition of the crushed rock is depicted in Table 1 .…”

Multielement Imaging Reveals the Diagenetic Features and Varied Water Redox Conditions of a Lacustrine Dolomite Nodule

“…Together with isotopic compositions of C and Sr, dolomite was thought to be formed in shallow water during the rapid fluctuation of the lake level, which was a result of the mixture of seawater and lake water. Given the presence of pyrite framboids and heavyδ 13 C of dolomite (up to 16‰), Gao et al asserted that the formation of dolomite was triggered by marine water intrusions, which not only brought Mg 2+ but also limited the precipitation of carbonate, even leading to fermentation of organic carbon in an anoxic environment [25]. Similar fermentation driven by methanogen was also supposed to have great contributions to the formation of the Permian lacustrine dolomite nodule in the Junggar Basin [19].…”

“…As a special type of sediment, lacustrine dolostones in the form of beds or nodules have been widely discovered in the Phanerozoic black shales, including the Middle Permian lake deposits in the Junggar Basin [19], the Later Cretaceous lake deposits in the Songliao Basin [25], and the Early Paleogene lake deposits in the Bohai Bay Basin [26]. These dolomites were supposed to be results of organic matter degradation through biotic and/or abiotic processes during the deposition of organic-rich black shales [19,25], whereas the causes that triggered the intermittent dolomitization during lacustrine anoxic events remained unknown.…”