Impact of diagenesis on the pore evolution and sealing capacity of carbonate cap rocks in the Tarim Basin, China

Analyzing the pore structure and sealing efficiency of carbonate cap rocks is essential to assess their ability to retain hydrocarbons in reservoirs and minimize leaking risks. In this contribution, the impact of diagenesis on the cap rocks' sealing capacity is studied in terms of their pore structure by analyzing rock samples from Ordovician carbonate reservoirs (Tarim Basin). Four lithology types are recognized: highly compacted, peloidal packstone-grainstone; highly cemented, intraclastic-oolitic-bioclastic grainstone; peloidal dolomitic limestone; and incipiently dolomitized, peloidal packstone-grainstone. The pore types of cap rocks include microfractures, intercrystalline pores, intergranular pores, and dissolution vugs. The pore structure of these cap rocks was heterogeneously modified by six diagenetic processes, including calcite cementation, dissolution, mechanical and chemical compaction, dolomitization, and calcitization (dedolomitization). Three situations affect the rocks' sealing capacity: (1) grainstone cap rocks present high sealing capacity in cases where compaction preceded cementation; (2) residual microfractures connecting adjacent pores result in low sealing capacity; and (3) increasing grain size in grainstones results in a larger proportion of intergranular pores being cemented. Four classes of cap rocks have been defined according to the lithology, pore structures, diagenetic alterations, and sealing performance. Class I cap rocks present the best sealing capacity because they underwent intense mechanical compaction, abundant chemical compaction, and calcite cementation, which contributed to the heterogeneous pore structures with poor pore connectivity. A four-stage, conceptual model of pore evolution of cap rocks is presented to reveal how the diagenetic evolution of cap rocks determines the heterogeneity of their sealing capacity in carbonate reservoirs.</p

Orbital forcing of tropical climate dynamics in the Early Cambrian

peer reviewedAccording to modern atmospheric circulation models, the Intertropical Convergence Zone (ITCZ), as the Earth's meteorological equator, plays an essential role in the low-latitude hydrologic cycles. The limited availability of high-resolution tropical climate archives, especially from the Early Paleozoic Era, severely limits our understanding of ITCZ migration dynamics in deep time. Here we present high-resolution climate-proxy records (i.e., magnetic susceptibility (MS) and Zirconium/Aluminum (Zr/Al)) from tropical marine sediments of the ∼526-million-year-old Qiongzhusi Formation in South China to investigate the link between orbitally forced insolation changes, ITCZ migration dynamics, and low-latitude climate processes. These orbital-scale variations in MS and Zr/Al series are interpreted as alternations between wet and dry cycles, controlled by monsoon intensity under the orbitally forced ITCZ-related paleo-Hadley Cell dynamics. Our results show that combined precession and obliquity orbital cycles had an impact on the Early Cambrian ITCZ migration. Specifically, the precession and obliquity forcing shift the mean position of the ITCZ latitudinally by changing the interhemispheric pressure contrasts, thus affecting the low latitude hydroclimate cycle. We report semi-precession cycles of 8.3–7.9 kyr, which were probably associated with the twice-annual passage of the ITCZ across the intertropical zone, consistent with the paleogeographical location of South China near the equator during the Early Cambrian. Observed ∼1.1 – ∼1.5 Myr eccentricity amplitude modulation (AM) cycles and ∼ 1.0 – ∼1.2 Myr obliquity AM cycles may provide geological evidence for the chaotic motion between Earth and Mars in the Early Cambrian

Extensional tectonics and sedimentary response of the Early–Middle Cambrian passive continental margin, Tarim Basin, Northwest China

The fact that several half-grabens and normal faults developed in the Lower–Middle Cambrian of Tazhong (central Tarim Basin) and Bachu areas in Tarim Basin, northwest China, indicates that Tarim Basin was under extensional tectonic setting at this time. The half-grabens occur within a linear zone and the normal faults are arranged in en echelon patterns with gradually increasing displacement eastward. Extensional tectonics resulted in the formation of a passive continental margin in the southwest and a cratonic margin depression in the east, and most importantly, influenced the development of a three-pronged rift in the northeast margin of the Tarim Basin. The fault system controlled the development of platform – slope – bathyal facies sedimentation of mainly limestone-dolomite-gypsum rock-saline rock-red beds in the half-grabens. The NW-SE trending half-grabens reflect the distribution of buried basement faults

Micropaleontology and palaeoclimate during the early Cretaceous in the Lishu depression, Songliao basin, Northeast China

Diverse and abundant microfossils, such as palynomorphs, algae and Ostracoda, were collected from lower Cretaceous strata of Lishu depression, located in southeastern Songliao basin, and were identified and classified in order to provide relevant, detailed records for paleoclimate research. The early Cretaceous vegetation and climate of southeastern Songliao basin have been inferred from the analysis of palynomorph genera, algae and Ostracoda of the LS1 and SW110 wells. The lower Cretaceous strata include, in ascending stratigraphic order, the Shahezi, Yingcheng and Denglouku formations. Palynological assemblages for each formation, based on biostratigraphic and statistical analyses, provide an assessment of their longitudinal variations. During deposition of the Shahezi Formation, the climate was mid-subtropical. Vegetation consisted of coniferous forest and herbage. During deposition of the Yingcheng Formation, the climate was south Asian tropical. Vegetation consisted mainly of coniferous forest and herbal shrub. In addition, fresh and saline non-marine water dominated the lacustrine setting during deposition of these formations. Deposition of the Denglouku Formation, however, occurred under a hot and dry tropical climate. The vegetation was mostly coniferous forest and lake waters became saline. Palaeoclimate variation is correlated by the lake level change and the development of sedimentary facies. Palaeoclimate contribute to the formation of hydrocarbon source rocks and reservoir

Provenance of Ordovician Malieziken Group, Southwest Tarim and Its Implication on the Paleo-Position of Tarim Block in East Gondwana

Tarim is inferred to have a close connection with East Gondwana during the Ordovician, but the position in East Gondwana remains controversial. In this study, we report 316 detrital zircons U-Pb data from three samples of Ordovician Malieziken Group sedimentary rocks, collected in the Qiate Section, Southwest Tarim, provided new insight into the position of Tarim in East Gondwana. Detrital zircons data indicated the maximum depositional age for the three samples is 489.5 Ma, 478.1 Ma, and 465 Ma, respectively, indicating the Qiate and Kandilike Formation of the Malieziken Group was deposited in Early—Middle Ordovician. The detrital zircons are characterized by two main peaks at ~490 Ma and ~1100 Ma, and three subordinate peaks at ~880 Ma, ~1400 Ma, and ~1650 Ma, suggesting most of the detritus of Malieziken Group from the South Kunlun Terrane (SKT) itself. However, the source of the ~1650 Ma peak is not found in the Tarim block, and the ~1400 Ma and the ~1650 Ma peak are absent in the middle of the three samples, which implied that there is an exotic source. The Paleoproterozoic sediment strata in the Albany–Fraser belt shows dominant peaks at ~1400 Ma and ~1650 Ma may have been transported to SKT and redeposited in the Malieziken Group during the Ordovician. The Malieziken Group shows detrital zircon age patterns resembling those of East Sumatra, Lhasa, and Western Australia which, in combination with the Albany–Fraser belt provenance, enables us to propose that the Tarim block has a close linkage with Western Australia, East Sumatra, and Lhasa in East Gondwana

Relationship between stylolite morphology and the sealing potential of stylolite-bearing carbonate cap rocks

We analyzed the sealing effectiveness of cap rocks bearing different types of stylolites using a combination of petrographic, petrophysical, pore structure, and sealing capacity characterization techniques. This study was based on examples of carbonate cap rocks that seal ultradeep hydrocarbon reservoirs of the Tarim Basin (China). Samples from both drill cores and their outcrop analogues were investigated to quantify how morphology influences the sealing capacity of different types of stylolite-bearing rocks. The study cap rocks consisted of mudstone, wackestone to packstone, grainstone, and dolomitic limestone. Four types of stylolites were identified: rectangular layer, seismogram pinning, suture and sharp peak, and simple wave-like types. The difference in the sealing capacity of carbonate cap rocks is attributed to their pore structure connections and the types of stylolites they develop. Samples bearing simple wave-like stylolites showed the best sealing capacity, followed by those with rectangular layer and suture and sharp peak types, whereas carbonates hosting seismogram pinning types had the lowest sealing capacity. The impact of stylolite segments on the rock sealing properties, however, differed from one segment to another. Rectangular layer−type stylolites could be divided into three distinct segments (with good, moderate, and poor sealing, respectively). Both the seismogram pinning and suture and sharp peak stylolite types were divided in two parts, with the former one having moderate and poor sealing and the latter exhibiting good and moderate sealing. The simple-wave−like type had a good sealing capacity all along the pressure-solution seam. The most effective sealing barriers for vertical fluid flow form when (1) calcite and siliceous cements are pervasively distributed in the vicinity of stylolites, forming highly cemented zones with lower porosity and permeability than their surrounding host rocks; (2) stylolites are enriched in insoluble residues; and (3) rare microfractures and dissolution vugs are found along the stylolites. This work provides useful examples for the prediction of the sealing potential of stylolite-bearing carbonate rocks according to stylolite morphology in other geologic settings

Supplementary document for Omnidirectional Color Shift Suppression of Full-color Micro-LED Displays with Enhanced Light Extraction Efficiency - 6306210.pdf

48601