Abstract
A comprehensive study of seep carbonates at the top of the organic- rich Maastrichtian to Danian Moreno Formation in the Panoche Hills (CA) reveals the mechanisms of generation, expulsion and migration of biogenic methane which fed the seeps. Two selected outcrops show 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 an aerobic oxidation of methane and carbonate dissolution. Secondly, after consumption of the oxygen, anaerobic oxidation of methane coupled with sulfate reduction triggered carbonate precipitation. Third, progressive diminishment of the methane seepage, lead to the deepening of the reaction front in the sediment and a lowering of precipitation rates. Carbonate isotopes, with δ13C as low as -51‰PDB, indicate a biogenic origin for the methane, while a 1D 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.
Highlights
Seep carbonates result from hydrocarbon migration up to the seabed at sites called cold seeps (Paull et al, 1984; Boetius and Wenzhofer, 2013)
The authors correlated seep carbonate distributions over several kilometers to the presence of underlying sand dykes emanating from the Panoche Formation, thereby providing evidence that the injectites served as hydrocarbon migration pathways
At least two major clusters of carbonate concretions were observed in direct connection with dykes (Figure 3), and these are the main focus of this paper
Summary
Seep carbonates result from hydrocarbon migration up to the seabed at sites called cold seeps (Paull et al, 1984; Boetius and Wenzhofer, 2013). They precipitate during the biocatalyzed anaerobic oxidation of methane (AOM) and by sulfate reduction in the sulfate–methane transition zone (SMTZ; Boetius et al, 2000). The authors correlated seep carbonate distributions over several kilometers to the presence of underlying sand dykes emanating from the Panoche Formation, thereby providing evidence that the injectites served as hydrocarbon migration pathways. A petroleum system, as defined by Magoon and Dow (1994), is the concept unifying the essential elements of petroleum geology (source rock, reservoir, seal, overburden), processes (generation, migration, accumulation), and that includes shows and seeps
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