Acid-insoluble residue/carbonate content of the Upper Cretaceous Niobrara Formation, Berthoud field, Denver basin, Colorado; a key factor for understanding natural fracturing in chalks?

Abstract

The acid-insoluble residue content and corresponding acid-soluble carbonate content of 148 samples from core representing the entire section of the Niobrara Formation was studied in the Berthoud State No. 3 well, Berthoud field, Larimer County, Colorado. The Smoky Hill Chalk Member is comprised of mostly impure chalks that, on average, contain about 30 wt % insoluble residue; basal Smoky Hill Chalk, however, contains purer chalk beds. The Fort Hays Limestone Member contains the purest chalk beds, some containing as little as 5 wt % insoluble residue. The data suggest that natural fracturing in the Berthoud State 3 well is related to basic bulk composition; contrast in bed composition and thickness of interbedded chalk and shale beds may be important in understanding natural fracturing within the Niobrara Formation. Acid-insoluble residue and carbonate contents of the Niobrara Formation in the Berthoud State 3 well identify cyclic variations that correspond to largeand small-scale alternations of chalk and shale. Large-scale cyclicity that produced the major chalk and shale zones in the Niobrara Formation is related to second-order eustatic changes within an overall major transgressive event. Similar large-scale cyclicity of organic matter content in a nearby well generally correlates with insoluble residue content. Smaller third-order cyclicity of beds, superimposed on the large-scale cycles, is related to climatic changes due to Milankovich-type variations in the earth's orbital patterns. Five natural fracture zones, four relatively minor in intensity, are identified in core of the Berthoud State 3 well. Each of the fracture zones occurs in the upper half of a thick chalk unit that is immediately overlain by a thick shale and each corresponds to the second order eustatic changes that define the main lithologic units of the Niobrara Formation. Fractures do not extend much into overlying shale sequences. The major and most intense zone of fracturing, showing evidence for extensive solution, fluid movement, displacement, and related deformation, occurs in the lower midportion of the formation. This 35-40 ft interval, informally referred to as fracture zone 3, contains several beds of chalk with relatively low ( 50 wt %) acid insoluble content. The second-order cycle curve for the Niobrara, as defined by the data, is shortened (thinner units) within this interval. This interval of major fracturing is also characterized by high resistivity. Fracture zone 3 is bounded by two large spikes on the gamma log at about 3,060 ft and 3,140 ft. Fracture zone 3, however, corresponds to a cyclic low in the organic matter profile for the nearby Berthoud State 4 well. The occurrence of natural fractures in Niobrara strata of the Berthoud State 3 well appear to be related to composition, as defined by acid-insoluble residue and carbonate contents, and correspond to intervals where strata have insoluble residue content in excess of 50 wt %, and where composition differences among the alternating strata over the interval approach 50 wt %. These differences or contast in bulk composition, as defined basically by insoluble residue and carbonate contents, probably produce differences in thermal and mechanical behavior of interbedded units during deep burial and stress that result in fractures; open, tensile fractures that are formed in extremely tight, deeply-buried, organic-rich chalks can create the porosity and permeability necessary for economic oil production.