Clastic Pipe Diagenesis of the Jurassic Carmel Formation, Southern Utah

Abstract

Well exposed clastic pipes that crosscut sabkha sandstones of the Jurassic Carmel Formation in southern Utah preserve multiple stages of diagenesis. Clastic pipes generally coarsen outwards with prominent calcite rhombs 0.2 - 1 cm long occurring in the coarse-grained rinds of the m-scale pipes. The calcite rhombs serve as records of fluid flow relevant to understanding the timing and extent of cementation within pipes, and its effects on connectivity in reservoir sandstones. Numerous dominantly monophase aqueous fluid inclusions of various sizes with some relatively large (40 μm) inclusions are present within the crystalline rhombs. Salinity measurements from the fluids all had relatively similar values spatially, with a brackish response falling within the 700-1400 ppm range. The monophase inclusions indicate low formation temperatures likely between 70–80 °C, which signify burial depths of less than a kilometer. In contrast, results from clay XRD reveal interstratified illite-smectite and kaolinite, which are present in both pipes and the host rock as clay cements. The interstratified illite-smectite has a reichweite ordering number of 1 (R1) and can therefore be termed rectorite, demonstrating at least several kilometers of burial. The clay data suggest a middle, mesodiagenetic phase during burial in contrast to the shallow diagenetic conditions implied from the calcite inclusions. The combinations of diagenetic phases show multiple stages of diagenesis within the clastic pipes as shallow, low temperature fluids moved through the pipes either: 1) pre-burial during early stage eogenesis or 2) after burial during late stage telogenesis. The fluid inclusion analysis indicates a brackish groundwater system with an evaporative component consistent with the sabkha interpretation for the host rock deposits. However, the well-developed crystal rhombs and lack of diagenetic overprinting support a late stage hypothesis. Large-scale sandstone intrusions in deep water systems are commonly targeted because of their potential as long-term fluid conduits in subsurface petroleum explorations. However, this Jurassic study shows that complex diagenesis can occur within clastic pipes, and the diagenesis should be an important consideration in the risk analysis and assessment of fluid history through time.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016