Occurrence of Siderite in the Point Mcintyre Field North Slope Alaska: An Indicator of Paleo Aquifers

James R. Boles, James J. Hickey, and Kevin Frank

Iron, sourced from weathering of shales of the Jurassic Kingak and Lower Cretaceous Miluveach Formations, was transported by fluvial water and coastal aquifers and fixed in nearshore fresh water-marine mixing zone as glauconite pellets and/or as siderite cement. The Neocomian section spent millions of years within 1000 feet of the surface, allowing for complex and extensive interaction between surface fluids and sediments. Siderite cement is most abundant in proximal facies that were exposed to meteoric water, and less abundant in fine-grained, well sorted more distal marine facies, which are isolated from meteoric input. Siderite cement associated with coarse-grained proximal facies is characterized by coarse-grained spar morphology, isotopically light oxygen, and Fe-ric composition. Siderite cement associated with distal marine facies is characterized by fine grain

scattered rhombic crystals, isotopically heavy oxygen, and Fe-Mg rich composition. Carbon isotopic composition of siderite reflects degree of openness of the sediment to oxygenated water. Fine-grained, poorly sorted sediment associated with pyrite contains siderite with positive carbon isotopic values (PDB scale), whereas well sorted, porous-permeable sediment has siderite with negative carbon isotopic values. Siderite with isotopically light oxygen is associated with oxidized and dissolved glauconite grains and kaolinite cement. Siderite formed from the sediment-water interface to burial temperatures of about 30°C, prior to the deposition of most of the overlying, laterally extensive, HRZ shale. ln all cases, siderite predates the formation of quartz overgrowths, which precipita ed at minimum temperatures of about 70°C.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California