Advantageous source–reservoir combination in siliciclastic–carbonate sedimentary systems in a saline lacustrine basin (Jiuquan Basin, NW China): Implications for prediction of tight oil enrichment

Unconventional tight oil reservoirs have recently emerged as a significant source of oil worldwide, and juxtaposed to organic-rich source rocks, tight mixed siliciclastic–carbonate sedimentary reservoirs are widely found. In China, such reservoirs are mainly found in saline lacustrine basins. In this paper, we conduct a systematic study on advantageous source–reservoir combinations (SRCs) in the Cretaceous Xiagou Formation in the Qingxi Sag of the Jiuquan Basin, a tight saline lacustrine and mixed siliciclastic–carbonate sedimentary oil reservoir. Mixed sediments result in multiple SRCs as follows: self-contained SRC, thick interbedded-type SRC, thin interbedded-type SRC, and down generated-up stored SRC. Geochemical characteristics of organic matter in the Cretaceous Xiagou Formation are established on the basis of Rock-Eval pyrolysis and total organic carbon (TOC) and vitrinite reflectance analyses. TOC (0.5–4.12%, av. 1.62%) and S2 (0.36–26.63 mg/g, av. 5.62 mg/g) indicate a good hydrocarbon generative potential. The organic matters contain predominantly Type II kerogen with minor contributions from Type I kerogen. The samples analysed have vitrinite reflectance in the range of 0.65–1.0%Ro, and pyrolysis Tmax in the range of 430–450 °C indicate that the source rocks are early mature to mature and within the main stage of oil generation. The tight oil resources are divided into 4 classifications according to TOC versus S1 diagram, and dolomitic mudstone in lower K1g3 and K1g1 is primarily considered to be potentially generative and productive. Reservoir characterization such as porosity, permeability, and pore-size distribution derived from nuclear magnetic resonance indicates that nano-meter-sized pores dominate pore size distribution, regardless of the lithology. Fractures contribute to the creation of relatively higher productivity reservoirs. The statistics of oil testing results shows that the daily oil productions increased basically from the down generated-up stored SRCs to thin interbedded-type SRCs to thick interbedded-type SRCs to self-contained SRCs. The movable oil saturations derived from nuclear magnetic resonance have the same changes as daily oil production. The self-contained SRCs and the thick interbedded-type SRCs are identified as advantageous SRCs that are important for tight oil prediction.

Zircon U–Pb dating and phase equilibria modelling of gneisses from Dinggye area, Ama Drime Massif, central Himalaya

The tectonic affinity of the Ama Drime Massif in central Himalaya remains a subject of debate. We provide evidences of the metamorphic conditions and new geochronological data for orthogneiss and paragneiss in the Riwu area, eastern side of Ama Drime Massif. The peak P–T conditions for garnet gneiss is 13–14 kbar and 750–765 °C using phase equilibria modelling. The zircon U–Pb dating results reveal that the orthogneiss has a crystallization age of 1,848 ± 12 Ma and zircon rims age of 1,767 ± 18 Ma. Three major age peaks characterize all detrital zircons from the paragneisses: 975, 1,161, and 1,749 Ma. These new data integrated with previous literature data indicate that (a) the Ama Drime Massif might be ascribed to the Greater Himalayan Series; and (b) the Himalayan terrane may have been located closer to both the India and Lhasa terranes during the amalgamation of the supercontinent Columbia.

Evolving stratigraphy of a Middle Triassic fluvial-dominated sheet sandstone: The Otter Sandstone Formation of the Wessex Basin (UK)

The Middle Triassic (Anisian) Otter Sandstone Formation of the Wessex Basin (UK) has long been considered a single undivided geological formation, but building on previous work, it is shown here to have a remarkably well-defined internal stratigraphy that is evident not only in the fluvial sedimentary architecture but also in the colour, grain size, sorting, and whole-rock geochemistry of the sandstones. The Otter Sandstone Formation is here divided into four named members of very different character (West Down Member, Otterton Ledge Member, Chiselbury Bay Member, and Pennington Point Member). The study highlights the importance of outcrop-to-subsurface correlation in stratigraphic studies, which suggests several major unconformities within the Otter Sandstone Formation. The Otter Sandstone Formation provides evidence for an increase in humidity throughout the early Middle Triassic, with an upward decrease in the proportion of calcrete and an apparent increase in channel size. Whole-rock major and trace geochemistry (chemostratigraphy) is widely used in the subdivision and correlation of red-bed sheet sandstones, and it is shown here that data, even when used in a very raw and unprocessed way, can be a useful stratigraphic discriminant. Fluvial sheet sandstones are often regarded as relatively homogeneous aquifer units, but this study shows that changes in fluvial style and background climate can introduce major changes to the geometry and physical properties of the aquifer or reservoir. The recognition of thin marker beds such as palaeosols can be particularly critical for the correlation of thick fluvial sheet sandstones.

Waves in lakes make waves in the Earth

Scientists at the University of Utah report that small seismic signals emanating from lakes can aid science. As a record of wave motion in a lake, they can reveal when a lake freezes over and when it thaws. And as a small, constant source of seismic energy in the surrounding earth, lake microseisms can shine a light on the geology surrounding a lake.