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Abstract: A 3D structural model of the Po Valley basin (Northern Italy) was built by integrating the dataset available from the public domain (DEM, wells, isobath-maps, cross-sections, outcrop-trends). The model shows the complex foredeep-foreland architecture across the basin, from the Moho level to the topography while illustrating the top Basement, top Triassic, top Mesozoic and base-Pliocene surface-grid structures. The results, by model slicing and isopach-map reconstruction, suggest that the deep Moho architecture and the original tectonics of the ancient Adria-Po Valley passive continental margin are key factors in controlling the current structures type, orientation and distribution, at any of the shallowest levels across the basin. In particular, the analysis of the final 3D Mesozoic geometries against the pre and post-Alpine trends confirms the structural interference between the mutually perpendicular Triassic–Jurassic extensional structures and the Tertiary compressional ones, this being evident from the regional to the oil-field scale. Despite the model uncertainty, mainly related to its dimension versus the original non-homogeneous dataset quality and distribution, the final geo-volume offers, for the first time in the region, a continuous three-dimensional visualization of the Po Valley tectonic architecture. It provides, simultaneously, a powerful tool for the reviewing of the basin structures and the potential support to future applications for both industry and academia. [Copyright &y& Elsevier]

Abstract: Reconstructing the evolution of paleofluid (over)pressure in sedimentary basins during deformation is a challenging problem, especially when no hydrocarbon-bearing fluid inclusions are available to provide barometric constraints on the fluid system. This contribution reports the application to a natural case (the Bighorn Basin) of recent methodological advance to access fluid (over)pressure level prevailing in strata during sub-seismic fracture development. The fluid pressure evolution in the Mississippian-Permian Madison–Phosphoria limestone reservoir is tentatively reconstructed from the early Sevier Layer Parallel Shortening to the Laramide folding in two basement-cored folds: the Sheep Mountain Anticline and the Rattlesnake Mountain Anticline. Results point out that supra-hydrostatic pressure values prevail in the limestone reservoir during most of its whole Sevier–Laramide history. The comparison of the reconstructed fluid overpressure values within situ measurements in various overpressure reservoirs in other oil-producing basins highlights that the supra-hydrostatic fluid pressure gradually reaches the lithostatic value during the whole basin contraction and fold development. During most of the LPS history, however, overpressure level can be defined by a mean gradient. Among the factors that control the pressure evolution, the mechanical stratigraphy, the stress regime under which fractures developed and regional fluid flow are likely dominating in the case of the Bighorn Basin, rather than classical factors like disequilibrium compaction or fluid generation during burial. A coeval evolution between fluid overpressure and differential stress build-up is also emphasized. The approach presented in this paper also provides estimates of strata exhumation during folding. [Copyright &y& Elsevier]

Characterizing the origin of natural fractures in organic-rich fine-grained deposits is key to constraining permeability evolution in these potential source rocks and tight reservoirs, as well as to assess the hydraulic connectivity of the fluid systems in which they develop. Differently oriented calcite-filled fractures (i.e. veins), hosted by organic-rich mudrocks of the Lower-Middle Triassic Montney-Doig unconventional resource play (Western Canada Sedimentary Basin), were sampled in sub-surface well cores from British Columbia. A multidisciplinary approach (including sedimentology, Rock-Eval pyrolysis, petrography, O–C–Sr isotope geochemistry and fluid inclusion microthermometry) was applied to host-rocks and fracture filling calcites. The results demonstrate the relevance and usefulness of such multidisciplinary studies to gather insights on: 1) the lithology-related factors controlling fracture occurrence; 2) the timing of fracture opening and the origin of the circulating paleo-fluids; 3) the openness of the fluid system through time. More specifically, host-rock facies (particularly grain size) and vertical facies changes appear to be the leading factors controlling fracture occurrence. A less relevant role was played by the occurrence of diagenetic carbonates, while TOC possibly did not control fracture occurrence. Three generations of calcite cemented fractures were identified. Vertical fractures (first generation) post-dated the onset of oil generation (Late Cretaceous). Horizontal, bedding-parallel fractures (second generation) post-dated the onset of gas generation and possibly opened close to maximum burial, corresponding to peak hydrocarbon (CH 4) generation (Late Cretaceous - Early Paleogene). Vertical fractures (third generation) post-dated the horizontal ones and opened during basin uplift (Middle to Late Paleogene). The consistent petrographic and geochemical features of all the calcite cements point to parent fluids in equilibrium with the host-rock lithologies, that possibly behaved as a closed hydraulic system during Late Cretaceous to Paleogene time; this would support the hypothesis that, at least in the portion of the basin investigated, the Montney Fm also acted as source rock of the unconventional system. • Fracture diagenesis of Montney-Doig Fms in sub-surface cores was characterized. • Lithology-related factors controlled fracture occurrence. • Timing of fracture opening and role of circulating paleo-fluids were constrained. • The succession behaved like a closed hydraulic system during Late Cretaceous - Paleogene time. [ABSTRACT FROM AUTHOR]

The paleostress history and associated deformation mechanisms affecting passive margins are seldom studied, as access to offshore parts is rather limited. We analyze an offshore wellbore core of the Albian, post-rift carbonates of the Sendji Fm which directly overlies the salt of the Aptian Loeme Fm in the Lower Congo Basin. Paleopiezometry based on stylolite roughness inversion (SRIT) and calcite twin inversion (CSIT) was combined with fracture analysis, U–Pb geochronology of calcite cement and burial modeling to unravel the orientations and magnitudes of horizontal and vertical stresses affecting the Sendji Fm over time, with a focus on the impact of salt tectonics on stress records. The results of SRIT on bedding-parallel stylolites constrain the range of depths over which the Sendji Fm strata deformed under a vertical principal stress σ1 to 650–2800 m (median ∼1300 m). Once projected onto the burial model derived from TemisFlow™ software, the range of depths converts to a period of pressure solution activity, from 101 to 15 Ma. Calcite twin measurements within the early diagenetic cement (U–Pb age = 101 ± 1 Ma) reveal three main types of stress regimes: (1) extensional stress regimes with σ3 trending ∼ N–S and ∼E-W associated with local, thin-skinned salt tectonics (101–80 Ma), (2) strike-slip stress regimes with σ1 trending ∼ NW-SE to NNE-SSW and a compressional stress regime with σ1 trending NNW-SSE, reflecting possible intraplate stress transfer from the distant Africa-Eurasia plate boundary (67–60 Ma); (3) a strike-slip and a compressional stress regime with σ1 trending E-W likely related to the mid-Atlantic ridge push possibly combined with effects of variations in elevation and density of the lithosphere and sedimentary flexural loading (15 Ma to present). The paleostress sequence suggests that the sedimentary cover was decoupled from the crust during salt tectonics and then recoupled from the late Cretaceous onwards. • The paleostresses affecting deep carbonates are characterized by studying an offshore core in the west African passive margin. • The timing of the successive stress regimes in the Sendji Fm is constrained. • Local gravitational stress from salt tectonics and far-field tectonic stress from plate boundaries influenced the margin. • The potential of a multi-proxy approach is established. [ABSTRACT FROM AUTHOR]

The timing of porosity destruction in reservoir rock is key for hydrocarbon exploration. This study constrains the porosity destruction of the syn-rift Barremian (130-125 Ma) TOCA Fm. from one drill core located in the Lower Congo basin (West African passive margin). We apply an unprecedented multi-proxy approach to reconstruct the absolute timing of mechanical compaction, pressure-solution and cementation as well as their relative contribution to porosity destruction. LA-ICP-MS U–Pb geochronology combined with petrological observations suggest that the main interstitial diagenetic cement C1 filling the porosity of the packstones and dated 127.4 ± 2.2 to 119.4 ± 6.4 Ma predates pressure-solution. The quantification of the cement distribution by image analysis indicates that C1 clogged ∼30% of the initial porosity before the strata were buried at 400 m. We assume that the early cementation limited the effect of mechanical compaction on porosity reduction which is estimated to ∼2% in the first 100 m of burial. In contrast, a first order estimate of the chemical compaction-reprecipitation can account for up to ∼6% of the initial porosity destruction, as estimated by the inversion of sedimentary stylolite roughness. This technique also documents that the burial-related pressure-solution in the TOCA Fm. occurred in the 550–1750 m burial depth range corresponding to a time spanning from 117 to 95 Ma considering the burial-time model of the TOCA Fm. Our study reveals that the initial porosity of carbonates was reduced down to its current value of 4–8% within the first 35 Ma of its burial history, reaching ∼10% after only 10 Ma, i.e. in the first 400–500 m of burial. This case study indicates that the fate of reservoir properties in bioclastic carbonate formations such as the TOCA Fm. may be largely controlled by early, very shallow diagenetic processes rather than subsequent deep-burial mesogenetic reactions. • The diagenesis of the tight, deeply buried Cretaceous lacustrine TOCA carbonates is characterized from the study of an offshore core in the Lower Congo basin. • The timing and amount of porosity destruction of the TOCA Fm. Are constrained. • The stylolite abundance and depth of stylolite activity in the TOCA Fm. Are controlled by texture. • The impact of early cementation on the reservoir properties of bioclastic carbonates is demonstrated. • The potential of a multi-proxy approach combining U–Pb geochronology of cements, burial modelling and inversion of the roughness of sedimentary stylolites is established. [ABSTRACT FROM AUTHOR]

Understanding the development of foreland-foredeep systems and the influence exerted by pre-existing structures on their evolution is an important step for defining the key factors that control long-term basin and lithosphere dynamics, comprehending the associated seismic hazard and assessing their economic potential in the domain of hydrocarbon exploration. The Po Valley is a rather unique foreland basin for two major reasons: a) it developed intermittently at the front of two different mountain chains, the Northern Apennines and the Southern Alps, progressively converging one towards the other; b) the inherited structures, mainly derived from the Mesozoic extensional tectonics, are oriented at high angle to the advancing belts. The coexistence of these two factors and their various implications make the Po Valley basin a complex case study that deserves attention. Taking advantage of the recent building of a 3D structural model across the region, we reconstructed the possible geometry and migration pattern of the Tertiary basins that developed at the front of the Northern Apennines and the Southern Alps, as part of the Po Valley tectonic evolution. In addition, a number of sections sliced from the 3D model across selected domains have then been used to restore the present-day structural units to their pre-compressional setting, while highlighting the key stages of their geological history. Results from the model analysis show that the Mesozoic extension-related tectonics and the associated carbonate facies geometry and distribution localized and constrained the Alpine structures inside/around the basin. Their control on the Cenozoic deformation and sedimentation is evident during the Paleogene and the Miocene whereas it becomes more subtle during the Plio-Pleistocene when lithospheric-scale mechanisms need to be invoked. Notwithstanding the model uncertainties and its explicit regional significance, our results may be taken as reference for any foreland-foredeep setting worldwide, especially in complex systems where tectono-sedimentary inhomogeneity is spatially and temporally dominant. [ABSTRACT FROM AUTHOR]