Stratigraphy of the Lorette Cave (Rochefort, Belgium): Study of the “gours suspendus” section

The Lorette Cave contains a wide variety of deposits within various stratigraphical contexts. This cave is a part of the complex underground meander cut-off of the Wamme and Lomme rivers, between some swallow-holes along their two talwegs near On, Jemelle and Rochefort, and the general resurgence at Eprave. The Lorette Cave is embedded within the Givetian limestone formations of the Calestienne. This cave displays the first part with a labyrinthic structure. Some parts of the cave galleries are affected by recent tectonic activity, which dislocates some galleries and provokes collapses. The second part of the cave comprises the West Gallery, which contains the most complete sedimentary series. The “gours suspendus” (hanging gours) section is located at the western end of the gallery. The cave contains numerous and rich detrital deposits. The oldest sedimentary unit is a diamictite found in several galleries (e.g. Galerie Fontaine-Bagdad, Salle du Cataclysme). It is composed of large decametric-sized quartz and sandstone pebbles coming from the erosion of the Lower Devonian formations of the Ardenne. This deposit is older than the U/Th dating limit, i.e. 350 ka. The West Gallery exposes an area of collapsed blocks and ends in a vast room. This gallery is filled with a thick fluvial series of upper Pleistocene age and capped by speleothems of Tardiglacial to Holocene age. The large terminal chamber is clogged by flooded pits. A tributary gallery shows a sedimentary series in a subsiding pit, the “Fosse aux Lions” (Lions’ Pit). These deposits are interstratified diamictite interbedded between two fluvial units, the upper part of which displays oblique stratifications. The dating of a summit stalagmite places this set at 120 ka. The present paper analyses a section made in the southern flank of the terminal room, close to the junction with the West Gallery: the “gours suspendus” section. A large part of this section consists of a complex fluvial deposit disconformably resting on top of a compact lower clay formation. This fluvial deposit is stratified, comprising mostly diamictites interstratified with thin levels of gravel and clay. It is capped by an upper clay unit and sealed by a flowstone. Thin strata of finer-grained size sediments (coarse sand), as well as clay lenses, occur within the lower clay. The diamictites indicate a torrential origin of the sediment. At the base, just above the lower clay, some sandy channelling strata testify that one or several fluvial deposition episodes occurred. Then, torrential and probably very short-living events are separated by decantation phases. The pebbles and smaller particles are made of quartz, sandstone and muscovite that most probably originated in the Lower Devonian formations. The “gours suspendus” section provides a new illustration of the succession of sedimentation and erosion phases in Belgian caves. It is now well demonstrated that speleothems grow mainly during temperate to hot and humid climatic phases and detrital infills are deposited in caves during cold/glacial phases. The physical erosion of sediments with ravine formations should be placed in the climatic history of the region. A gullying by a coarse detrital formation like that of the new section is due to a powerful heavy loaded current. The deposits within caves were therefore available, which can only occur during a cold phase due to the absence of continuous vegetation cover. The sand and clay levels interstratified between levels of pebbles indicate nevertheless distinguished flow regimes. However, this torrential lava in the new section seems different from the old diamictite. The deposition of the sedimentary units in the West Gallery seemingly happened during a glacial–interglacial transition. This sedimentological study sets a future perspective for dating flowstones and stalagmites at the top of or embedded within the deposit levels in order to propose a more robust chronological frame for the evolution dynamics of the cave infilling of the Lorette Cave in relation to the climatic history of the region.

Discerning In-Situ Performance of an Enhanced-Oil-Recovery Agent in the Midst of Geological Uncertainty: II. Fluvial-Deposit Reservoir

Summary An enhanced-oil-recovery (EOR) pilot test has multiple goals, among them to be profitable (if possible), demonstrate oil recovery, verify the properties of the EOR agent in situ, and provide the information needed for scaleup to an economical process. Given the complexity of EOR processes and the inherent uncertainty in the reservoir description, it is a challenge to discern the properties of the EOR agent in situ in the midst of geological uncertainty. We propose a numerical case study to illustrate this challenge: a polymer EOR process designed for a 3D fluvial-deposit water/oil reservoir. The polymer is designed to have a viscosity of 20 cp in situ. We start with 100 realizations of the 3D reservoir to reflect the range of possible geological structures honoring the statistics of the initial geological uncertainties. For a population of reservoirs representing reduced geological uncertainty after 5 years of waterflooding, we select three groups of 10 realizations out of the initial 100, with similar water-breakthrough dates at the four production wells. We then simulate 5 years of polymer injection. We allow that the polymer process might fail in situ and viscosity could be 30% of that intended. We test whether the signals of this difference at injection and production wells would be statistically significant in the midst of geological uncertainty. Specifically, we compare the deviation caused by loss of polymer viscosity with the scatter caused by the geological uncertainty using a 95% confidence interval. Among the signals considered, polymer-breakthrough time, minimum oil cut, and rate of rise in injection pressure with polymer injection provide the most-reliable indications of whether a polymer viscosity was maintained in situ.

Research on Sedimentary Environment of the Wayaobao Formation in Ordos Basin

Wayaobao Formation of late Triassic, in Ordos Basin is composed of clastic rocks, such as grey green thick-bedding medium-coarse-grained lithic arkose, lithic feldspathic sandstone, fine-medium-grained lithic feldspathic greywacke, quartzose feldspathic sandstone, et al. Detailed study of Wayaobao Formation in Shenmu was carried out about lithology, sedimentary structures, biological characteristics, heavy mineralogy and trace element characteristics, then a conclusion was drawn that Wayaobao Formation in Shenmu, Ordos Basin was fluvial deposit.