Abstract

About 150 samples from core representing nearly a complete section of the Niobrara Formation in the Berthoud State No. 3 well, Berthoud Field, Larimer County, Colorado, were analyzed for insoluble residues, carbonate content, and bulk-rock and clay mineralogies. The Smoky Hill Chalk Member contains mostly impure chalks averaging about 30 wt% insoluble residue; purer chalks having up to 95% carbonate are in the Fort Hays Limestone Member. Cyclic variations in insoluble residue:carbonate ratios of the Niobrara correspond to large- and small-scale cycles of chalk and shale or marl deposition. Large-scale cyclicity produced the major chalk and shale zones in the Niobrara. Similar large-scale cyclicity of organic matter content in a nearby well generally correlates with insoluble residue content. Smaller-scale bed cyclicities are found throughout the Niobrara Formation and may be related to climatic changes contemporaneous with ashfall deposition from periodic volcanism.

The acid-insoluble fraction of the Niobrara mainly consists of clay minerals, quartz, pyrite, feldspar, and organic matter. On average, clay minerals constitute 55 wt% of the insoluble residue. Illite and ordered mixed-layer illite/smectite comprise more than 90 wt% of the clay minerals. Much of the clay and quartz are terrigenous detrital components that are cyclic along with insoluble content and probably also reflect eustatic and climatic changes.

Extensive diagenetic changes have occurred in the chalks and shales of the Niobrara Formation in the Berthoud Field area during progressive burial to maximum depths that were probably in excess of 8,000 ft (2,400 m). Diagenetic changes include the maturation of organic matter producing indigenous oil, chemical compaction reducing porosity to less than 10% and increasing brittleness of chalk, and the conversion of plastic, smectite clays to more brittle, illitic clays.

Five natural fracture intervals are identified in core of the Berthoud State No. 3 well. Each fractured interval occurs in thick chalk units or benches enclosed by thick calcareous shale or marl that correspond to the largest bed cycles. Minor, hairline, calcite-filled fractures occur in four of the intervals. These thin fractures are mostly concentrated in the upper half of the large chalk benches and do not extend into the overlying marls. Fracturing commonly corresponds to intervals where bed compositional contrast is at maximum and where chalks with relatively low (less than 20 wt%) acid-insoluble content are interbedded with calcareous shale or marl having high (greater than 50 wt%) acid insoluble content. The above relations suggest that these fractures have formed by contrasts in mechanical properties of chalk and marl beds produced by contrast in bed composition(s).

A heavily fractured interval (informal interval 3) having open oil-stained fractures is bounded by two faults. This 35 to 40 ft (10 to 12 m) interval occurs within the informally designated “C chalk” of the the Smoky Hill. Interval 3 is characterized by anomalously high resistivities (greater than 100 ohm-meters) indicative of high oil saturation. Faulting created the open fractures, solution textures, bed offsets, and brecciation within interval 3, producing major fractures more favorable for oil production.

Internal controls on the mechanical strength of chalk include: 1.) depositional factors such as bed composition, contrast, and thickness; and 2.) diagenetic factors such as porosity and bulk density. These internal controls probably determined initial fracture occurrence and extent within the Berthoud State No. 3 well when regional stresses from external sources were applied. A later external factor, primarily faulting, was probably the cause of major fracture development in the Berthoud State No. 3 well. Although faulting produced fractures favorable for more economic oil production in this well, composition may play a more significant role in controlling fracture development and extent in other chalk reservoirs of the Niobrara Formation.