Diagenetic complexities of iron oxide cements in Mesozoic sandstones of Utah, U.S.A.

<p>Stratigraphic units of the Colorado Plateau comprise a remarkable Mesozoic section in Utah. Thse units are ideal for studying sandstone diagenesis where there is established basinal context of depositional facies and tectonics, as well as continuity of exposure. To untangle the complex relationships and diagenetic histories, it is crucial to understand host rock properities (porosity and permeability), authigenic mineralogies (that give clues to fluid composition), diagenetic textures, and age dating. This study is a review and synthesis of previous work that has contributed to the understanding of the diagenetic history recorded in authigenic iron oxide precipitates. We discuss cement generations and mineralogies, fluid chemistries, origins and mobilization of iron, and timing of precipitation. Spheroidal cemented mineral masses (concretions) are common within many Mesozoic units of Utah – most notably the Jurassic Navajo Sandstone.  However, formation of these concretions is still not completely understood. Spheroidal concretions are currently a “hot topic”, especially since the discovery of similar “blueberry” features on Mars with their implications for habitability, and the potential for these nodules to host biosignatures. Several models for spheroidal concretion formation are evaluated. Understanding how iron is mobilized and precipitated and how spheroidal concretions form have implications for similar geometries and mineralogies in many terrestrial regions, but will require continued integrated studies across multiple scales (see Baker and Potter-McIntyre, this session). These scales include the submicroscopic levels of understanding and detecting the potential role of microbes in mineral precipitation, to the larger scale mapping of regional diagenetic coloration and mineral patterns that could represent records of basinal fluids and the response to climate, tectonics, and regional hydrology.</p>

DIAGENESYS STAGE ANALYSIS OF SANDSTONE INTERVAL ON WELL DAR-24, GABUS FORMATION, ANOA FIELD, WEST NATUNA BASIN

Stage of diagenesis of a rock will effect the quality of the rock as a reservoir. Hence, it is a necessary to analyze the diagenesis stage of sandstone at Anoa Field, West Natuna Basin, since the diagenesis stage has not been identified properly. The analysis is carried out using thin section method in five different depths. The product of diagenesis is observed for its cementation level, compaction, recrystallization, dissolution, replacement, and type of porosity wich developed in the rock. The appearance of quartz overgrowth cement and pore filling and pore lining calcite cement show a diagenesis stage which are recrystallization and cementation. The appearance of bent mica mineral and suture grain contact can be a sign of late stage compaction. Dissolution of matrix, cement and grain in the sample show that the rock has come to mesodiagenesis stage. As a result of the observation, the conclusion can be made that the rock has passed the eodiagenesis and mesodigenesis phase. Porosity of the section is both primary porosity (interparticle) and secondary porosity (dissolved) with a range 15%-20% (medium to good). Crossplot depth vs porosity show a linear distribution, which when the depth is increase the porosity will decrease. So it can be concluded that the process of diagenesis is very influential on the quality of reservoir rocks in the study area.Keyword: Gabus Formation, sandstone diagenesis, stage of diagenesis.