Spatial and geochemical characterization of anomalous, map-scale carbonate bodies in the Monterey Formation, Santa Maria Basin, California

Abstract

In the Santa Maria Basin (SMB) of central California, four carbonate bodies have been mapped in the lower members of the Monterey Formation, proximal to faults, or where the Monterey Formation unconformably overlies older strata. These bodies are large (kilometer scale, in some cases) and mappable on a 1:24000 scale, making them unique carbonate occurrences within the Monterey Formation. Typical carbonate occurrences are nodules, veins, replacements of other lithologies, and diagenetic laminations and beds, which are observed on millimeter to meter scales. Most of these atypical carbonate occurrences can be distinguished from primary depositional calcareous lithofacies that occur locally in the basal Monterey. They are characterized by dolomitized and silicified host rock containing breccias and cement that often include euhedral quartz, and in some cases, are associated with a distinct internal zonation and the presence of hydrocarbons. These characteristics are consistent with hydrothermal origin as found in some other occurrences in the SMB. Hydrothermal dolomite (HTD) deposits also occur in mid-continent North America and other locations worldwide. Such deposits are often hosts to valuable mineral resources and serve as hydrocarbon reservoirs, making them economically significant deposits. Polished slabs and thin sections reveal a complex, generally brecciated fabric within the carbonates that have been microsampled for X-ray diffraction, trace element, and oxygen and carbon isotopic composition analyses. These data will provide insight into genetic relationships between carbonate occurrences, including ranges of temperature of formation and compositional variation between locations. Understanding the depositional nature of the atypical carbonate bodies in the SMB can provide insight into the burial, dewatering, chemical, and tectonic histories of the SMB, provide a potential new hydrocarbon play concept (HTD play) for the area, and may explain several anomalously productive wells that have been drilled in the past.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016