Stratigraphy, sedimentology and petrophysics of transgressive tight gas sandstones, Almond Formation, Wyoming




Merletti, German Diego

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With the recent increase in development of unconventional reservoirs, the ability to predict rock quality from sedimentary and petrophysical models has become paramount to the development of tight gas sandstones. In this way, a refined understanding of the primary sedimentary, stratigraphic and diagenetic controls on rock quality permits more reliable hydrocarbon distribution prediction and more economical drilling programs. The Almond Formation in southwest Wyoming is characterized by three depositional facies associations (DFA); shoreface, delta and fluvial/coastal plain, which present three distinctive porosity-permeability trends. Differences between petrophysical facies are primarily driven by diagenetic (cementation and grain dissolution) effects on different framework grain compositions. Depositional textural variation, such as grain size and sorting is minimal in all DFAs. This research focuses on building an understanding of the transgressive deposits by studying the variability of sandbody types, comparing and contrasting their reservoir architecture in a setting with a well-documented back-stepping stacking pattern. Construction of a high-resolution chronostratigraphic framework, in 1,450 wells over 6,200 km², revealed the evolution of fundamental fine-scale architectural elements. Detailed analysis and integration of cores and well logs along a spectrum of sandbodies document stratigraphic evolution from longshore accretion to seaward progradation associated with progressively increased infill of a shrinking lagoon. End members sandstone geometries include: 1) narrow, finger-like sandstone morphologies with well-developed lagoonal facies and; 2) broad, strandplain-type sandbodies with coastal plain-dominated back-barrier. This research also addresses a problematic aspect of tight gas reservoirs: the prediction of rock-quality-dependent water saturation (SW) models with depth. Primary drainage and imbibition saturation-height models (SHM) were developed from special core analysis and integrated with porosity and permeability logs to verify the SW state of reservoirs. Assuming that reservoirs were fully charged with hydrocarbons, the drainage SHM is key for flagging departures from the expected rock-quality-dependent water saturation. Observations in tens of wells show that the reference resistivity-derived saturation can be predominantly fitted by primary drainage SHM. However, some upper Almond shoreface bars that have anomalously high SW can be fitted with primary imbibition saturation functions. These fitting exercises indicate that some Upper Almond reservoirs imbibed due to trap tilting or leaking through outcrops.


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