Mechanisms of biogenic gas migration revealed by seep carbonate paragenesis, Panoche Hills, California

Abstract

A comprehensive study of seep carbonates at the top of the organic-rich Maastrichtian to Danian Moreno Formation in the Panoche Hills (California) reveals the mechanisms of generation, expulsion, and migration of biogenic methane that fed the seeps. Two selected outcrops show that seep carbonates developed at the tip of sand dykes intrude up into the Moreno Formation from deeper sandbodies. Precipitation of methane-derived cements occurred in a succession of up to 10 repeated elementary sequences, each starting with a corrosion surface followed by dendritic carbonates, botryoidal aragonite, aragonite fans, and finally laminated micrite. Each element of the sequence reflects three stages. First, a sudden methane pulse extended up into the oxic zone of the sediments, leading to aerobic oxidation of methane and carbonate dissolution. Second, after consumption of the oxygen, anaerobic oxidation of methane coupled with sulfate reduction triggered carbonate precipitation. Third, progressive diminishment of the methane seepage led to the deepening of the reaction front in the sediment and the lowering of precipitation rates. Carbonate isotopes, with d 13 C as low as -51‰ Peedee belemnite, indicate a biogenic origin for the methane, whereas a one-dimensional basin model suggests that the Moreno Formation was in optimal thermal conditions for bacterial methane generation at the time of seep carbonate precipitation. Methane pulses are interpreted to reflect drainage by successive episodes of sand injection into the gas-generating shale of the Moreno Formation. The seep carbonates of the Panoche Hills can thus be viewed as a record of methane production from a biogenic source rock by multiphase hydraulic fracturing. A U T H O R S

What makes seep carbonates ignore self-sealing and grow vertically? The role of burrowing decapod crustaceans

“…The repeated succession of early AOM-related cements in Aurel is in many respects comparable to that of seep carbonates of the Moreno Formation (Paleocene, Great Valley Basin, California) that records pulsed methane supply from a deep reservoir (Blouet et al, 2017). In fact, the succession of cements in the Aurel PBH shows, like in the Moreno Formation, an evolution from smaller (microsparite) to larger (sparite) crystals and presence of radiaxial sets of crystals possibly corresponding to the fans of aragonite needles of the 580 Moreno Formation (Blouet et al, 2017), and from the Terres Noires Formation of Beauvoisin (Peckmann et al,1999). The interpretation of the cement succession in the Moreno Formation was also based on the presence of corrosion surfaces that indicate episodic changes in fluid chemistry or temperature, which was not observed in Aurel.…”

“…What are now the parameters that control vertical growth of the PBH in sections where background marls only have <5 cm interbedded limestone beds? The role played by Thalassinoides burrows in focusing methane migration has been identified by Wiese et al (2015) and Blouet et al (2017). This study helps 610 understand the specific factors that make it possible for this type of burrows to remain open in spite of likely overprinting by shallower burrow tiers.…”

“…Leaving aside the effects of late 535 silicification, the cement infill of Phase 3 burrows and their host sediment is then quite simple: what is recorded is AOM-related calcite precipitation in the SMTZ, expressed by micrite-scale crystals in the porous network of homogenized micrite, microspar-size crystals in the porous network of pellet grainstone and in the first 0.1-0.5 mm above burrow walls (BMSpar-CMSpar) and spar-size crystals (Spar-1) in the remaining free space of the lumen. The radiaxial sets of grey carbonate crystals that occasionally terminate the sequence of cements (RAx) 540 could be recrystallized equivalents to the aragonite fans described by Blouet et al (2017) in the Panoche Hills (CA, USA) or by Peckmann et al (1999) in Beauvoisin. Calcite crystal continuity across the BMSpar-CMSpar contacts indicates that the brown color simply reflects the inclusion of the filamental bushes into the growing microspar.…”

The Formation of Forced Folds and Wing‐Like Sand Intrusions Driven by Pore Fluid Overpressure: Implications From 2‐D Experimental Modeling

“…Since sand intrusion bodies can provide pathways for fluid flow through low‐permeable strata and change fluid pressure distribution in a basin (Andresen, ; Blouet et al, ; Briedis et al, ; Cartwright et al, ; Jonk et al, ; Schwartz et al, ), understanding of their evolution and formation mechanisms is of high economic importance, for example, for exploration of hydrocarbon reservoirs (Duranti & Mazzini, ) or risk assessment of CO 2 storage sites (Rutqvist, ; Torabi et al, ). The formation of large‐scale (several hundreds to a few kilometers in size) SI likely requires a supralithostatic pore fluid overpressure in a source layer of undercompacted sand encased in low‐permeable, cohesive sediments (Cartwright, ; Jolly & Lonergan, ).…”

Formation of linear planform chimneys controlled by preferential hydrocarbon leakage and anisotropic stresses in faulted fine-grained sediments, offshore Angola

“…Flow directions in the subsurface and the distribution of hydrocarbon leakage sites at the seafloor are preconfigured by such preexisting structures (Thrasher et al, 1996;Moore et al, 1990). The morphology of structures formed during fluid leakage records the style and intensity of fluid expulsion and is thus useful for deciphering the fluid migration history (Roberts et al, 2006;Blouet et al, 2017;Imbert et al, 2017;Imbert and Ho, 2012;Ho et al, 2012bHo et al, , 2018. As 3-D seismic reflection data have played an increasingly important role in visualisation and identification of fluid flow features (Heggland, 1997), by conducting seismic analyses for vertical successions of fluid leakage expressions around faults, such as gas chimneys feeding pockmarks and seep carbonates, it is possible to unravel the timing and pathways of migrating fluids and the sealing efficiency of faults (Ligtenberg, 2005;Plaza-Faverola et al, 2012;Ho et al, 2016).…”