ALBERT

Description: Dissolved fluoride was determined for pore waters at eight sites drilled on Hydrate Ridge during Ocean Drilling Program (ODP) Leg 204 and one site drilled in the Peru Trench during ODP Leg 201. All nine sites contain a shallow (〈20 m) sulfate-methane transition (SMT) above abundant methane including gas hydrate. For Sites 1248, 1249, and 1250 on the crest of Hydrate Ridge, F- concentrations are significantly lower than that of seawater in the shallowest samples (〈50 µM), rise to a broad maximum, and generally decrease with depth. The low values at the top are consistent with rapid F- removal at or near the seafloor, and the relatively smooth F- profiles are consistent with high upward fluid fluxes. In contrast, Sites 1244, 1245, 1247, 1251, and 1252 from the flanks and slope basins of Hydrate Ridge and Site 1230 from the Peru Trench have F- profiles apparently characterized by two lows with an intervening high. Processes involving sediment components appear to consume F- at shallow depth, release F- at intermediate depth, and consume F- again at deeper depth. The upper low in F- concentrations consistently lies near the SMT where pore water alkalinity and Mg2+ profiles suggest precipitation of Mg-rich carbonate. A similar pattern occurs at other sites drilled into methane-charged sediment. We speculate that Mg-rich carbonates (e.g., high-Mg calcite, protodolomite, and dolomite) remove F- from pore water near the SMT but, with burial and recrystallization, return F- to pore waters at depth. Authigenic Mg-rich carbonates conceivably represent a major sink of F- from the ocean, although additional work is needed to confirm this idea.

Description: Leg 201 of the Ocean Drilling Program (ODP) focused on understanding subsurface microbial communities and their influence on the chemistry of the surrounding environment (D'Hondt, Jørgensen, Miller, et al., 2003, doi:10.2973/odp.proc.ir.201.2003). During the cruise, sediment cores were collected from four different marine environments of the eastern Pacific Ocean: deep open ocean beneath the moderately productive upwelling regime of the equator (Sites 1225 and 1226), shallow waters of the highly productive Peru shelf (Sites 1227-1229), a deepwater gas-charged zone in the Peru Trench (Site 1230), and a deep open-ocean area under oligotrophic waters (Site 1231). Each of these sites had been drilled previously (during either Deep Sea Drilling Project [DSDP] Leg 34, ODP Leg 112, or ODP Leg 138), although not for detailed microbiological or geochemical investigations. Interstitial waters are routinely analyzed for major cations aboard the JOIDES Resolution drillship. An exception was made during Leg 201 because other dissolved species (e.g. Fe2+, Mn2+, Ba2+, and acetate) were of more immediate interest and because low-resolution profiles of major cations already existed from the earlier cruises (D'Hondt, Jørgensen, Miller, et al., 2003, doi:10.2973/odp.proc.ir.201.2003). Consequently, major cation analyses were withheld for shore-based work. We present here the Ca2+, Mg2+, K+, Na+, and Sr2+ concentrations for pore waters from six of the sites, Sites 1225-1230. Pore waters from Site 1231 are not included in this report because of data inconsistencies.

Description: The production of crude oil from resource plays has increased enormously over the past decade. In the USA, around 63% of total output in 2019 was from unconventional production. The major unconventional plays in the USA (e.g., Permian Basin, Anadarko Basin, Eagle Ford, etc.) have become some of the world’s largest oil producers. However, unlike “conventional” exploitation, the target zones in unconventional systems are generally the source rocks themselves or adjacent strata and require numerous horizontal wells and stimulation via hydraulic fracturing to meet production targets. In order to maximize production, operators have developed various well stacking methods, all of which require some form of monitoring to ensure that well spacing is optimized and fluid production is not being “stolen” from adjacent formations, thereby reducing the production potential in associated wells. This necessity, amongst other geochemical considerations related to source rock characterization, has resulted in the expansion of “production allocation” and “time lapse geochemistry” methods. These methods were initially developed for conventional production decades ago, but have since been adapted to unconventional systems. However, the direct applicability of this method is not straightforward and numerous considerations need to be taken into account, foremost among which are: (1) “What defines your end-members?” (2) “Are these end-members valid across a meaningful development area?” and (3) “What is the most appropriate use of geochemistry data in these systems?”. Reservoir geochemistry studies, which include both “time lapse geochemistry/production monitoring” and “production allocation”, are valuable geochemical methods in unconventional plays but need to be used appropriately to provide the cost savings and business direction that operators expect. In this paper, we will discuss a number of case studies, both theoretical and natural, and outline the important factors which need to be considered when designing a reservoir geochemistry study and the common pitfalls which exist. The case studies and best practice approach discussed are designed to highlight the power and flexibility of geochemical data collection methods, integration with the operator’s knowledgebase, and other analytical methods to customize the program for individual development programs. Emphasis is placed upon developing robust and applicable fluid relationships from geochemical data and evidence for statistically significant changes through time.