In recent years, significant breakthroughs have been made in the exploration of both conventional and unconventional oils in the Chang 9 Member of the Upper Triassic Yanchang Formation in the Longdong area. However, the origin of hydrocarbons remains contentious, largely due to insufficient characterization of the source rocks in both the Chang 7 and Chang 9 Members. In this study, the geochemical characteristics and hydrocarbon generation potential of these two members were comprehensively analyzed. The Chang 7 Member is characterized by the development of thick mudstones with high total organic carbon (TOC = 0.25–19.89 wt.%) and significant hydrocarbon generation potential (S 1 +S 2 = 0.29–93.12 mg/g; HI = 72–714 mg/g TOC). In contrast, the mudstones in the Chang 9 Member are relatively thin and exhibit lower organic abundance (TOC = 0.11–4.79 wt.%) and subordinate generation potential (S 1 +S 2 = 0.02–6.53 mg/g; HI = 33–262 mg/g TOC). Biomarker and isotope analyses indicate that the superior hydrocarbon potential and Type II₁ kerogen of the Chang 7 Member are attributable to its highly anoxic lacustrine facies and a mixture of plankton and terrestrial plants. Conversely, the Type II₂ kerogen in the Chang 9 Member, characterized by slightly weaker generation potential, reflects a weakly anoxic deltaic environment with a higher proportion of terrestrial plants. This study provides a comprehensive understanding of the Chang 7 and Chang 9 source rocks and offers an effective theoretical basis for further research on the petroleum system of the Yanchang Formation in the Longdong area.
Exploration decisions rely on pre-drill volume estimates that shape drilling choices, portfolio ranking, and economic appraisal. This study evaluates the reliability and accuracy of such forecasts and tests whether simple calibration can improve them. Using prospect-level forecasts from 1998–2022 on the Norwegian Continental Shelf, we compare pre- and post-drill volume distributions and assess forecast quality using complementary diagnostics of reliability, accuracy, and uncertainty. Results reveal systematic optimism and miscalibration, as realized volumes fall too often in the lower tail of predicted distributions and uncertainty bands are generally too narrow, with little improvement over time. We then develop an operational calibration framework consisting of (i) a reference class benchmark based on historical analogs and (ii) simple distribution-level adjustments to central tendency and quantiles. Reference class adjustment provides the most stable improvements while maintaining forecast sharpness. In contrast, methods that enforce probabilistic reliability mainly widen uncertainty, highlighting a trade-off between reliability and focus. Overall, the outside view, particularly the reference class benchmark, offers a simple, low-cost improvement to exploration workflows and supports routine updating.
Fractal geometry theory is often used to characterize the heterogeneity of pore-throat structures; however, some pitfalls that are often ignored will lead to incorrect conclusions. To provide insights into these issues, mercury intrusion porosity experimental data for the Chang 2 sandstone reservoirs in Jingbian area and the Chang 8 tight sandstone reservoirs in the Qingcheng area were used; and the impact of calculation equations, data ranges and the distribution on fractal dimension ( D f ) values, as well as the correlation between the fractal dimension and permeability, were analysed. This analysis shows that different calculation equations will result in quite different fractal dimension values. Meanwhile, for a specific equation, different cumulative orders of the accumulated mercury saturation ( S Hg ) will result in completely different D f values, and so the equations used need to clarify the cumulative order for S Hg . It is also found that the limitation of scale affects the fractal dimension values, determining whether the fractal curve exhibits unifractal or multifractal characteristics. The fractal curves of different types of samples show that small pore throats, especially nanoscale pore throats with radii less than 100 nm, are commonly not fractal; this is due to nanoscale pore throats being close in size and so have no significant orders of magnitude. In addition, for multifractal sandstone reservoirs, when the radii intervals of large pore throats are not consistent with the radii intervals of permeability contribution, the correlation between permeability and the fractal dimension of large pore throats may be not strong.
The Clair Field is thought to contain the largest hydrocarbon accumulation on the UK Continental Shelf, with the principal reservoir comprising low-matrix-permeability, naturally fractured, Devonian clastic red beds of the Lower Clair Group (LCG). The pre-2020 development plan on the Clair Ridge focused on achieving production rates via targeting natural fractures. However, recent challenges with open-fracture prediction (from early water breakthrough to low productivity wells) have led to a renewed focus on understanding the matrix. This study summarizes the predictability of the matrix character across the LCG, specifically to: (1) optimize well placement (matrix property trends); (2) understand completion requirements (rock-strength trends); and (3) improve pre-drill deliverability predictions and well sequences. The study integrated a range of reservoir quality analysis with sedimentology, core observations, petrophysical logs and heavy mineral ratios. The analysis unveiled a relationship between the resulting sedimentological characteristics (facies), diagenetic overprint, primarily calcite cement and the contrasting styles of reservoir quality across the LCG units that denotes fluctuating climatic conditions. Furthermore, constraining spatial trends in clay mineral volumes (source of calcite cement) within a resolved depositional model has produced a pre-drill predictive capability in cement volumes and facies (matrix character). The availability of a predictive model for the matrix character has increased confidence in well-performance prediction and impacted well sequencing, reservoir targeting and completions strategies. Together with a successful production-increasing drilling strategy, this has increased production on the Clair Ridge from 40 to 80 Mboed (thousand barrels of oil equivalent per day) in 2021 alone.
Petrographic analyses of forty-five volcanic rock samples from seven wells in the Santos Basin were conducted to investigate lithofacies distribution, eruptive styles, and petroleum system implications from the Aptian Barra Velha Formation to the Turonian– Maastrichtian Itajaí-Açu Formation. Six lithofacies were identified, including coherent basaltic lava flows, autobreccias, peperites, and lithic, vitric, and crystal tuffs. Lava flows composed mainly of plagioclase and augite (± olivine), commonly displaying amygdales and secondary alteration to chlorite, zeolites, calcite, and palagonite, dominate the early pre-breakup intervals. Younger post-breakup units are characterized by abundant volcaniclastic deposits and intense palagonitisation. Integrated petrographic and stratigraphic data indicate a temporal transition from predominantly subaerial Hawaiian and Transitional volcanism during the rift and post-rift stages to subaqueous Surtseyanstyle hydromagmatic activity during the drift phase, strongly influencing reservoir heterogeneity, porosity evolution, and seal development within the basin.
