Abstract: Heron North Field, Navajo Nation, San Juan County, Utah: A Case Study for Small Calcarenite Carbonate Buildups

CHIDSEY, THOMAS C., JR. and Eby, DAVID E.

Mapping of the regional lithofacies of the Pennsylvanian (Desmoinesian) Paradox Formation, Desert Creek zone, within the Paradox basin of southeastern Utah indicates a relatively untested belt of shallow-shelf, calcarenite carbonate deposits. This narrow but long belt of calcarenite lithofacies is between open marine lithofacies and the margins of intra-shelf, salinity-restricted lithofacies. Calcarenite buildups represent high-energy environments where shoals and/or islands developed from shallow-marine processes on the shelf.

The Heron North field in the Navajo Nation is an excellent example of the type of fields which potentially lie within this 20-ml-long lithofacies belt. The field consists of one well, the North Heron No. 35C, completed in 1991 at an IPF of 605 BOPD and 230 MCFGPD. The field is a lenticular, northwest- to southeast-trending linear mound/beach complex, 0.8 mi long and 0.5 mi wide. The reservoir consists of a bioclastic calcarenite deposit above an anhydrite- and salt-plugged phylloid-algal buildup. This deposit developed in a carbonate-beach to foreshore environment. The 60-ft-thick reservoir consists of alternating 2- to 4-ft-thick packages of uniform beach calcarenite and poorly sorted foreshore and storm-lag rudstone or breccia deposits. An 8-ft-thick anhydrite lies directly above the reservoir interval and creates an effective seal.

The principal reservoir rocks in the field are porous (15%), sucrosic, dolomitized grainstones and packstones above tight bafflestones composed of stromatolithic mats. Pores are often lined with bitumen which in many instances plugs pore throats. The calcarenite and bafflestone intervals are separated by low-permeability, dolomitized wackestones and mudstones.

Cumulative production from the Heron North field is 203,148 BO and 0.32 BCFG. Estimated primary recovery from the field and others that are likely along the trend is 990,000 BO and 2.65 BCFG.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado