Understanding and mapping variability of the Niobrara Formation across Wattenberg field, Denver Basin

Abstract

Wattenberg Field has been a prolific producer of oil and gas since the 1970s, and a resurgence of activity in recent years in the Niobrara Formation has created the need for a detailed study of this area. This study focuses on mapping regional trends in stratigraphy, structure, and well log properties using digital well logs, 3D seismic data, and core X-ray diffraction data. Across Wattenberg, the Niobrara is divided into the Smoky Hill Member (made up of A Chalk, A Marl, B Chalk, B Marl, C Chalk, C Marl, and Basal Chalk/Marl) and the Fort Hays Limestone Member. Directly beneath the Niobrara, the Codell Sandstone is the uppermost part of the Carlile Formation. Stratigraphic trends in these units are primarily due to differential compaction and compensational sedimentation. The largest structural trend is a paleo-high that runs east-west to northeast-southwest across the middle of the field. It has a relief of about 100 ft, and is 20 mi wide. The A Chalk and A Marl show evidence of submarine erosion over this area. Faults mapped from 3D seismic data are consistent with previously published data on a proposed polygonal fault system in the Denver Basin. Faults are 1000-10,000 ft long, have 30-150 ft vertical displacement, about 45° dip, and commonly form grabens 20-1000 ft wide. The faults and grabens have a northeast-southwest trend and are commonly seen en echelon. This orientation is interpreted to be the result of transtension. Total organic carbon (TOC) was calculated from resistivity and density logs and mapped across Wattenberg Field. The high range of maturity limits the accuracy of the calculation, especially in the chalks. Mean TOC values are erroneously high at the edges of the field, where maturity is lower. An anomalous resistivity low is recognized and mapped in the center of the field. Its primary cause is attributed to overpressuring as a result of higher thermal maturation. Petrophysical analysis on the well logs and geophysical analysis on the 3D seismic data are recommended for future work.