Interbedded Source and Reservoir Rocks in a Hybrid Tight-Oil Petroleum System: Mississippian Heath Formation, Central Montana, USA

Abstract

The late Mississippian Heath Formation is the primary target of an emerging tight oil play. The organic-rich beds of the Heath are well known as the primary oil source rocks for more than 100 MMBO of oil produced from the Tyler Sandstone and other reservoirs in central Montana, however, recent geological work, coring, and horizontal drilling has exposed the Heath Formation as an unconventional tight oil reservoir sourced by interbedded organic-rich shales and carbonates. Parts of two cores, the Cirque Rock Happy #33–3H and the Cirque Lucky Strike #10–4H, provide examples of source and reservoir rocks in the 450 foot-thick Heath. Reservoirs in the Heath range from dolomite to limestone to calcareous shale. Larger pores are most common in intertidal to supratidal dolomites with up to 18 percent porosity; fractures and microporosity are present in all lithologies. The best reservoirs are developed in the Heath Limestone interval of the Heath. The stratigraphically lowest source rock interval in the Heath is informally called the Van Dusen zone. It contains a mixture of coals and high-TOC calcareous mudstones; oils generated by these sources contain abundant terrestrial biomarkers. The coals in the lower parts of the Heath indicate a warm, humid climate. The middle source interval in the Heath is the Cox Ranch Oil Shale which contains interbedded dark brown to black calcareous mudstones with TOC up to 26 wt%. The Heath Limestone is above the Cox Ranch and includes source beds with TOC up to 11 wt%. Anhydrites, intertidal to supratidal dolomites, and the presence of gammacerane in the generated oils indicate an arid climate and hypersaline waters. The Cirque Rock Happy #33–3H core is located in the southern part of the Big Snowy Trough, has two dolomite intervals, and is in the peak oil thermal maturity window (Tmax 440 degrees C). The corresponding horizontal well was drilled in a dolomite in the lower part of the Heath Limestone, had an IP of 271 Bopd, and produced 36 degree API gravity oil. The Cirque Lucky Strike #10–4H core is in the northern part of the Big Snowy Trough, is closer to the lowstand basin, and has thinner dolomite pay development. This well is near the onset of oil generation (Tmax 431 degrees C); the corresponding horizontal well was drilled in dolomites of the lower Heath Limestone, had an IP of 104 Bopd, and produced 29 degree API gravity oil.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015