POSSIBLE OCCURRENCES OF TRANSITION ZONES AND RESIDUAL OIL ZONES BELOW OIL-WATER CONTACTS IN MATURE OIL FIELDS,

SOUTHEAST SASKATCHEWAN

Chengyu Yang and Hairuo Qing University of Regina

Adopted from Melzer, 2006

The gradational nature of the transition zone (TZ) is interrupted by a thick, relatively constant, low oil saturation zone—Residual oil zone (ROZ)

Oil saturation profiles of ROZ. (a) average oil saturation profile adopted from a Wasson Denver Unit well in the Permian Basin (modified from Koperna and Kuuskraa, 2006). (b) average oil saturation profile adopted from Seminole San Andres Unit in the Permian Basin (modified from Trentham et al., 2010, 2012). (c) water saturation profile adopted from the Nubian sandstone reservoir in the Zei Bay field of the Gulf of Suez, Egypt (modified from Elshahawi, 1999).

Transition Zone & Residual Oil Zone

TZ forms under capillary force ROZ occurs under certain geological and hydrodynamic conditions, and may be recoverable using tertiary recovery techniques

Illustration of possible origins for ROZ (modified from Melzer, 2006)

Origins of ROZ

Melzer (2006) proposed three possible origins for ROZ :

Type 1 ROZ is formed from the original oil trap subject to post-entrapment regional tilt.

Type 2 ROZ is created when the original oil escapes due to the breaching of its upper seal.

Type 3 ROZ results from a change of hydrodynamic conditions in the underlying aquifer.

Objective

Perform a preliminary investigation of possible occurrence of the TZ/ROZ in southeast Saskatchewan.

• Reviewed geological history related to ROZ

• Identified 4 existing pools that possibly have TZ/ROZ potential.

- Well log - Core samples - fluorescence test of cuttings - DSTs and etc.

• Estimated the potential of the possible TZ/ROZ

Map of major oil pools in the Williston Basin, southeast Saskatchewan (GeoSCOUT database).

Rosebank Alida pool

Bellegarde Tilston pool

Frys Tilston-Souris Valley pool

Un-named Souris Valley pool

A typical basement subsidence profile of the Williston Basin generated at well location 6-9-7-13W2 (modified from Olajide and Bend, in press).

Geological Background Related to ROZ—The geological background of the Williston Basin might have preserved the possibility of ROZ

Water driving force of the representative Mississippian Madison Group aquifers in Southeast Saskatchwan, showing the water flows generally travel from west to east (modified from Palombi, 2008)

• Mayor oil producer—the Madison Group, oil generated and migrated during the Late Cretaceous – the Early Tertiary time.• Basin being experiencing intermittent tectonic uplifts since the Late Cretaceous• Subsurface water flows in the Mississippian strata might be result of Laramide Orogeny (Late Qretaceous) and Laurentide ice-sheet retreat

(Quaternary).

Oil and water saturation analysis using well log data

Oil saturation calculated from well logs in the Bellegarde Tilston pool area suggesting the occurrence of a TZ/ROZ below the traditional OWC.

An interval about 30 meters thick with oil saturation ranging from 0% to more than 40% is present from the Lower Tilston to the uppermost Souris Valley

ROZ/TZ

Exemplary Pickett plots adopted from the Bellegarde Tilston pool area

Exemplary Pickett plots adopted from the Frys Tilston-Souris Valley pool area

Pickett crossplots

Separations between the resistivity of different zones illustrate that the resistivity of the possible TZ/ROZs is, to some degree, affected by hydrocarbons.

Evidences on cores from possible ROZ

Cores from the Alida Bed in Well 4-14-5-32W1, the Rosebank Alida pool.

• DST below OWC recovered only water

• Oil stains below OWC

• Core derived oil saturation (about 10%) below OWC (GeoSCOUT database)

• Fractures and dissolutions that may indicate occurrence of water flow(s)

Cores from the possible TZ/ROZ in dry hole 4-36-73-1W1, the Frys Tilson-Souris Valley pool area

Cores from the possible TZ/ROZ in dry hole 9-17-6-31W1, the Bellegarde Tilston pool area.

Fluorescence test of drill cuttings

Fluorescence test of Well 4-33-008-31W1, 990m ~1080m, every 5 meters, adopted from the un-named Souris Valley pool area.

Fluorescence of Well 16-33-4-32W1, 3640 ft ~ 3730 ft (1109.5 m ~1137 m), every 5 feet.

Strong fluorescence test agrees with the oil saturation profile, on the presence of oil for about 40 meters below the OWC

Strong fluorescence indicates the presence of oil between the OWC and bottom seal.

Tilted OWC As suggested by Hubbert (1953), the accumulations of oil under hydrodynamic conditions will invariably exhibit tilted oil-water interface. This can be described by the equation below:

Cross section across the Rosebank Alida showing titled OWC

Where:dz/dx = slope of the oil-water contactw = density of the watero = density of the oildh/dx = potentiometric gradient

Sulfur water (H2S in water) and Oil shows reported by DST in dry holes in surrounding non-producing areas

Well locations in the Rosebank Alida pool and surrounding areas where DSTs recovered sulfur water and/or water with hydrocarbon shows.

H2S is the most conspicuous and abundant product of bacterial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR)

The paleo-temperature of the Mississippian Mission Canyon Formation in the Williston Basin had never reached the threshold for TSR (100 oC) but might be optimum for BSR (20 oC ~ 40 oC).

BSR sulfur water may be indicative of water flow and ROZ

A typical basement subsidence profile of the Williston Basin generated at well location 6-9-7-13W2 (modified from Olajide and Bend, in press).

Never high enough for TSR

Inversed oil saturation

Post-entrapment water flows may be responsible for this. It swept through the zone below OWC, taking away initial oil accumulation. Porous reservoir tends to lose oil more easily.

• Above OWC, high oil saturation in the porous reservoir above OWC

• Below OWC, oil saturation is lower in porous reservoir than in tighter reservoir.

Usually, the porous reservoir is likely to be more permeable than tighter reservoir, and thereby tends to have higher oil saturation.

Oil saturation profile of well 14-28-8-31W1, un-named Souris Valley pool area.

Preliminary Estimation of the Oil Reserves within Identified TZ/ROZ

Pool* OOIP in MPZ (MMbl)adopted from GeoSCOUT database OIP in TZ/ROZ (MMbls)

Bellegarde Tilston 13.8 7.7

Frys Tilston-Souris Valley 36.4 20.6

Rosebank Alida 126.4 61

Total 176.6 89.3

Estimates of OIP of TZ/ROZ

* The un-named Souris Valley pool is not included because necessary pool information (such as producing area) is not available.

Discussions and Conclusions

• The basin underwent post-entrapment uplifts (Olajide and Bend, in press) and hydrodynamic flows (Palombi, 2010) that may induce changes of the geological conditions of existing oil pools, and thus create ROZ.

• 4 pools with TZ/ROZ potential are identified and the thickness of identified TZ/ROZ is up to 47 meters. Evidences include log-based oil saturation profiles, fluorescence tests, cores from possible TZ/ROZ, sulfur water and tilted OWC.

• The OIP of TZ/ROZs was estimated to be approximately half the amount of the oil within the associated MPZs.

• No conclusive interpretation can be made on the origin of the identified TZ/ROZ, although tilted OWCs, sulfur water, dissolutions and “inversed” oil saturation profile may imply that the identified oil bearing zones are possibly ROZ related to water flows.

• There may be other ROZs undiscovered.