Integrated geochemistry, organic petrology, and sequence stratigraphy of the Triassic Shublik Formation, Tenneco Phoenix # 1 well, North Slope, Alaska, U.S.A.

Abstract

The Tenneco Phoenix #1 well recovered continuous core through the Shublik Formation, allowing an uncommon opportunity to comprehensively study organic and lithostratigraphic facies variations in a known hydrocarbon source-rock interval. The lower portion of the cored Shublik Formation represents part of a transgressive systems tract (TST) and is characterized by alternating well-laminated and bioturbated shales. A pronounced condensed section occurs at the top of this interval. The Shublik Formation above the maximum flooding surface (MFS) represents initial deposits of a highstand systems tract (HST); it contains coarser grained material and is more bioturbated than the base of the unit. The distribution of oil-prone facies within the Shublik Formation is dependent on stratigraphic position and is predictable within a sequence stratigraphic framework. The transgressive portion is dominated by oil-prone facies while the upper, regressive portion is predominatly gas-prone to non-source. Well-laminated, organic-rich, oil-prone intervals in the Phoenix core correspond to uranium anomalies on the spectral gamma ray log. An organic petrologic study of 54 core samples from the Shublik Formation is consistent with log analysis and source-rock geochemistry. Both subtle and dramatic organic facies changes characterize the TST. Fluorescent amorphinite, marine alginite, and other exinites are the dominant kerogen macerals. Subtle variation in the maceral concentration of this oil-prone organic debris is typical of this interval; however, there are four major changes in kerogen composition in this transgressive interval. These changes include fluctuations in kerogen composition that reflect likely variations in those factors that controlled deposition and preservation of the organic matter in the Shublik Formation. There is another significant reduction in the amount of oil-prone kerogen in the condensed section at the top of the TST. Non-fluorescent amorphinite is the dominant organic matter type. In general, fluctuations in kerogen composition are much less pronounced in the HST when compared with the TST. Fluorescent amorphinite is the dominant kerogen type, but there is also 10–15% vitrinite with comparable amounts of inertinite. The amount of these maceral types gradually increases stratigraphically upward in the HST. Because of this, source-rocks in this interval tend to be gas-prone. However, there is no significant change in the apparent origin of the organic matter immediately across the MFS; there is still considerable kerogen of marine origin in the HST.