Stratigraphy of Palaeocene phosphate pelagic stromatolites (Prebetic Zone, SE Spain)

The hemipelagic domain of the ancient southern continental margin of Iberia is home to a strongly condensed pelagic succession (6–15 cm thick) characterized by the presence of phosphate stromatolites. This succession, probably generated in the slope of the continental margin, records a period of some 9 Ma, corresponding to the latest Maastrichtian to Late Thanetian interval. A microstratigraphical analysis allows for characterizing and biostratigraphically dating six successive developmental stages in the succession, which outline the main environmental evolution of the depositional setting. The Wrst of them determined the generation of a submarine hardground during the latest Maastrichtian to earliest Danian interval. The other Wve are represented by Wve successive microstratigraphical, unconformity-bounded, genetic units, respectively Early– Middle Danian, Late Danian–Early Selandian, intra-Selandian, Late Selandian–Early Thanetian, and Middle–?Late Thanetian in age. The three oldest units are characterized by the accretion of phosphate stromatolites, favoured by very low rates of pelagic sedimentation and by a microbially mediated extra input of phosphate. The two youngest units are dominated by carbonate deposition, which has always taken place at very low rates. Condensed sedimentation was abruptly interrupted at the end of the Palaeocene (?latest Thanetian), when the condensed succession and its hosting substrate were gravitationally slumped and redeposited at the base of the slope in the form of a megadebris Xow that can be now observed in Sierra de Aixorta (Alicante, SE Spain). The Aixorta pelagic phosphatic stromatolites are among the youngest ever described, and their existence suggests that the oceanographic conditions necessary for their development prevailed during most of the Palaeocene, but disappeared during the Late Selandian, never to return

Carbonate secondary porosity development in a polyphase paleokarst from Precambrian system: upper Sinian examples, North Tarim basin, northwest China

Carbonate karst is one of the research highlights in the field of carbonate reservoir geology. High porosity zones can develop in carbonate rocks which are modified by the karst. However, the carbonate porosity development and its evolution process in a polyphase paleokarst are rarely reported in the geological record. Here, the paper focuses on this point from the Precambrian Upper Sinian carbonate in the north Tarim basin, northwest China. Five outcrops and two wells reveal that the karstic porosity development of Upper Sinian carbonate was petrography-controlled of microbial dolostone and grain dolostone. The karstic porosities were in relation to three paleokarst phases: (1) eogenetic karst during depositional environment, (2) telogenetic karst at the end of late Sinian, and (3) hydrothermal karst in the early Permian. Eogenetic karst was related to frequent and short sea-level fluctuation, and was characterized by selective dissolution of intragranular dissolution porosity, moldic porosity, bird’s eye porosity, and intergranular dissolution porosity. Telogenetic karst was related to tectonic uplift, and was characteristic of non-selective dissolution of vugs and cavities with karst intensity increasing upward. Hydrothermal karst was related to geothermal events, and was featured by highly solution-enlarged porosities which mainly overprinted earlier telogenetic and eogenetic karst porosities. The suggested paleokarst model, combined with the related karstic porosity evolution process, will perfectly reveal its porosity development

Later fluid alteration of eogenetic karst spaces in carbonate: insights from the Cambrian Longwangmiao Formation, Northwestern Sichuan Basin, China

Pore-cave systems formed by karstification in the eogenetic stage of carbonate rocks provide abundant potential reservoir space for hydrocarbons. However, whether these dissolution pore-caves can become effective reservoir spaces during the later burial period, serving as the key to the success of hydrocarbon exploration. Therefore, it is important to explore the fluid activities and their alteration effects on eogenetic karst reservoirs during the later burial. Focusing on the Cambrian Longwangmiao Formation in the northwestern Sichuan Basin, this study systematically analyzed the formation of reservoir space in the eogenetic stage and the reworking of the system by fluids in the later stages, based on petrology, geochemistry, burial history, and tectonic evolution data. Results showed that many millimeters to several centimeters scale of pores and caves in the Longwangmiao Formation were produced by eogenetic karstification. These pore-caves underwent by two episodes of dolomite infilling in the shallow burial stage (D1) and in the Caledonian–Hercynian period (D2). Geochemical parameters indicate that D1 and D2 were both affected by meteoric water. In the early shallow burial stage, the dolomitic fluid was enriched in a relatively closed, reducing environment, whereas in the later stage, the fluid was affected by a relatively open oxidizing environment due to Caledonian–Hercynian fractures. Both D1 and D2 took place before the massive hydrocarbon migration from the Cambrian source rocks in the Middle Permian to those of the Middle Triassic. After the formation of the dissolution pore-caves, the precipitation from two episodes of dolomitic fluids led to the degradation of the Longwangmiao Formation carbonate reservoir space in the northwestern Sichuan Basin. In the southern part of the Shatan section-Well MS1, closed to the paleo-uplift of the central Sichuan Basin, where eogenetic karstification was superimposed by Caledonian–Hercynian supergene karstification, may be form effective reservoir and is a signific