LOCATING NIOBRARA FRACTURES-2 (CONCLUSION) ANALYZING RESISTIVITY, OIL PRODUCTION OF NIOBRARA IN WYOMING'S SILO FIELD

Reed A.Johnson, R. Timothy Bartshe
Energy Foundation Inc.
Lakewood, Colo.

The Silo field area in 15n-16n/64w-65w, Laramie County, Wyo., serves as an excellent study area for Niobrara oil production in that it contains 47 vertical wells that have produced nearly exclusively from Niobrara (Fig. 7).

No other field in the Denver basin produced from such a large number of wells and only from the Niobrara. Statistically, Silo field is the best available analog for potential future Niobrara oil fields. Through June 1991, 10 horizontal wells have been drilled and completed in the Niobrara in the Silo field area (Fig. 9, 10A, 10B).

The field was discovered in December 1980 by Amoco Production Co. with the completion of the Champlin 300 Amoco B1 in SE SE 5-15n-64w.

In early 1983, after marginal production recovery from the Smoky Hill C chalk, the Amoco 1-B Leroy Goertz, SW NE 31-16n-64w, was recompleted in the B chalk of the Smoky Hill member.

Following recompletion, the well was initially capable of flowing more than 600 b/d of oil and subsequently produced 178,000 bbl of oil and approximately 90 MMcf of gas in the following 17 months, an average of approximately 350 b/d.

As of December 1990, cumulative production was 263,511 bbl of oil and 145.839 MMcfd, or 288,655 bbl of oil equivalent at 5.8 cu ft/bbl.

By spring 1985, 47 vertical wells had been completed in the Niobrara. Cumulative production from vertical well bores as of June 1990 was 1,315,000 bbl of oil and 957,000 MMcf, or 1,481,000 bbl of oil equivalent.

The distribution of production in Silo field is characterized by few wells with high cumulative production and many wells with low cumulative production (Fig. 7). Of 47 completed vertical wells, only two wells or 4% have produced more than 100,000 bbl/well.

These two wells have produced a combined 477,593 BOE, or 32% of the cumulative field production.

Four wells, or 9% of the total, have produced 50,000-100,000 bbl/well and account for 22% of cumulative field production.

Nine wells, or 19% of the total, have produced 25,000-50,000 bbl/well and account for 19% of cumulative field production.

The other 31 wells, or 66% of the total, have produced less than 25,000 bbl/well and account for 27% of the cumulative field production.

No spatial production trends are readily apparent with the exception of a northeast-southwest trend of four wells in 31 - and 29-16n-64w, which contains the two most productive wells ("Goertz trend").

The present structure of the base of the Niobrara (Fig. 8) in the Silo field area dips westward at approximately 60-80 ft/mile.

The field itself is associated with a very subtle shift in the dip direction from west to west-southwest, and an increase in dip to approximately 90-100 ft/mile. The trend of this slight structural anomaly is approximately west-northest. No prominent structural closures or noses are evident.

Without the high density of well control available in Silo field, this feature would likely not be evident from subsurface mapping. Based on seismic interpretation, the structural anomaly has been related to an apparent boundary of dissolution of Permian salt underlying the Niobrara.19

Based on the thickness of various stratigraphic intervals, the majority of the salt dissolution apparently occurred prior to the end of Niobrara deposition. A small amount may have occurred as late as Upper Cretaceous, during deposition of the lower and middle Pierre shale.

The fact that the salt dissolution edge coincides with the slight structural anomaly in Silo field may suggest some salt dissolution occurred after Niobrara deposition; however, whether the salt dissolution is a causative factor in the localization of Silo field or a coincidental result of another factor, such as movement association with an underlying basement feature, is unclear from this data.

Porosity of the Smoky Hill chalks is relatively uniform throughout the field area, ranging from 7-12% (computed density porosity). No apparent relationship exists between porosity and production trends.

Although porosity in the Niobrara may be expected to consist of two components, matrix porosity and fracture porosity, no distinction between the two is evident between productive and nonproductive intervals. This reflects either that fracture porosity is not significant relative to matrix porosity, or that fracture porosity is not detected by most porosity logs.

Resistivity of the Smoky Hill chalks is the only subsurface parameter that generally correlates with the distribution of production (Fig. 9). Using the Smoky Hill B chalk as an example, all productive wells exhibit maximum resistivity of at least 35 ohm-m, which defines the approximate limits of the field.

Three areas of maximum resistivity of 100 ohm-m or greater are evident, one associated with the two most productive wells in the field, the Amoco 1-B Goertz, SW NE 31-16n-64w, and the Exxon 2-C Goertz, NE SW 31-16n-64w.

Maximum resistivity within the field ranges up to 300 ohm-m. Maximum resistivity outside the field is observed as low as 20 ohm-m.

Even though the limits of the field may be delineated by maximum resistivity, the magnitude of the resistivity does not directly correlate to cumulative production from each well, and the mapped trends reveal only general characteristics. The correlation of high resistivity to the productive limits of the field indicates the resistivity tools are responding to oil bearing fractures.

Application of the threshold resistivity concept to Silo field provides confirmation of the technique. Corrected formation temperature in the Silo field area approximates 185 F. Ten vitrinite reflectance measurements in the Niobrara from the Davis Oil Co. 1 Berry, in 13-16n-66w,8 approximately 7 miles west-northwest of the center of Silo field, average 0.76% reflectance with a standard deviation of 0.09%.

Rounding the Ro to 0.8%, the calculated threshold resistivity for Silo field is 35 ohm-m at formation temperature, equal to the empirically observed minimum productive resistivity.

The triangulated resistivity anomaly (TRA) method of resistivity calculation more clearly defines high resistivity trends than mapping of maximum resistivity (compare Fig. 9 and 10A). TRA values range from zero (outside the field) to more than 6,000 ohm-m-ft. The value in the Amoco 1-B Goertz exceeds 3,500 ohm-m-ft.

Inasmuch as anomalously high resistivity is interpreted as indicating proximity to oil bearing fractures, the mapped anomalies may be expected to exhibit some degree of linearity. In fact, an orthogonal linearity is evident in the TRA interpretation.

Narrowly confined northeast-southwest trends are evident, particularly those intersecting 31-16n-64w ("Goertz trend"), 5-15n-64w ("South Prince trend"), 25-16n-65w, and 26-16n-65w.

Each of these trends contain TRA values in excess of 2,000 ohm-m-ft. The Goertz trend includes the best producers in the field, and the South Prince trend includes the well reporting the highest initial production rate in the field.

Perpendicular to the northeast-southwest linear trends are broader, west-northwest/east-southeast trends of resistivity anomalies (Fig. 10B). The most prominent is that intersecting the northeast-southwest Goertz trend, which is coincident with the proposed Permian salt dissolution edge. Other parallel west-northwest/east-southeast trends are also apparent.

The orthogonal pattern of resistivity anomalies indicates two fracture systems, one oriented northeast-southwest and the other oriented west-northwest/east-southeast (Fig. 10B). The west-northwest/east-southeast trends appear to be more widespread than the more confined northeast-southwest trends.

The west-northwest/east-southeast orientation is consistent with the direction of well bore elongation indicated by four arm dipmeter data and the dominant, inferred fracture orientation in the Goertz trend area as interpreted from experimental 3D, 3-C seismic.19

The northeast-southwest orientation is consistent with observed patterns of production interference in the Goertz trend and the more strongly defined resistivity anomalies.

A crossplot of TRA vs. cumulative production per well further supports the interpretation of a dual fracture system (Fig. 11).

Two distinct trends are apparent. The first is represented by a "low productivity envelope" defined by an upper limit that represents increasing cumulative production with increasing TRA. The maximum cumulative production in this envelope is approximately 60,000 BOE/well.

However, for any given value of TRA, cumulative production may range from the upper limit of the envelope to minimal volume. The majority of Silo field wells falls within the envelope.

A "high productivity" trend is characterized by a linear increase in cumulative production with increasing TRA to a cumulative production limit of 286,000 BOE/well. The high productivity trend contains only five wells, including the two best producers which are situated on the northeast-southwest Goertz trend.

The pronounced duality expressed in the TRA vs. cumulative production crossplot supports the interpretation of two fracture orientations. The "low productivity envelope" represents well bores in communication with the more pervasive west-northwest/east-southeast oriented fracture systems, which are interpreted to consist of a widespread network of small, poorly interconnected fractures (hairline or microfractures?).

The "high productivity trend" is interpreted to represent the contribution of the northeast-southwest fracture systems. These systems are more narrowly confined and likely consist of more open, widely communicated fractures that may serve to connect the fractures of the west-northwest/east-southeast trends.

The high productivity trend likely represents well bores in communication with a northeast-southwest fracture set alone or with the intersection of a northeast-southwest fracture set and a west-northwest/east-southeast fracture set.

Well bores that communicate only with the west-northwest/east-southeast fracture system may exhibit variable TRA values and initially produce at high rates but, due to the limited volume of the interconnected fracture system, will decline precipitously and yield relatively little cumulative oil.

A well recording a very high TRA may be in very close proximity to a substantial oil bearing fracture system (i.e., the northeast-southwest fracture system) but not produce significant quantities of oil due to the failure of the northeast-southwest trend to communicate with the well bore. The more pervasive nature of the west-northwest/east-southeast fracture systems as contrasted with the more narrowly confined nature of the northeast-southwest systems partially explains the interpretation of others that the west-northwest/east-southeast is the dominant fracture trend.

The west-northwest/east-southeast systems are probably dominant in terms of fracture density but not in terms of the magnitude of the fracture pore system available to a well bore. Statistically, the west-northwest/east-southeast fracture systems will be intersected by far more vertical well bores than the northeast-southwest systems.

To date, most horizontal wells drilled in Silo field area have been oriented to intersect the west-northwest/east-southeast fracture systems. This may account for the encouraging initial results of many of the horizontal completions and the subsequent rapid production declines.

No horizontal wells to date have clearly intersected a northeast-southwest fracture system in Silo field, with the possible exception of the Gerrity 4-9H State, in 4-15n-64w (Fig. 10B), which reportedly flowed 27,000 bbl of oil in its first 56 days of production.

In evaluating the relationship between resistivity response and cumulative production, it is critical that the distinction be made between the nature of the two types of data.

In a fracture mediated reservoir, production data reflect the degree to which a fracture system is in communication with the well bore and the magnitude of the communicated pore system.

In contrast, the resistivity tool detects a property within a volume of rock surrounding the well bore but not necessarily in communication with it. In the case of a deep induction tool (i.e., 6FF40), the tool reads a radius surrounding the well bore of more than 15-20 ft.

If the resistivity tool is responding to oil bearing fractures within the radius of investigation of the tool, the fractures need not be in communication with the well bore to be detected. Production data and resistivity response may not correlate due to this difference in the origin of the data.

SUMMARY

Resistivity in Niobrara chalks increases in a predictable pattern with initial burial, the onset of thermal maturation, and continued burial and maturation.
Threshold resistivity defines the resistivity associated with the matrix oil saturation. It is dependent on the level of thermal maturation and present formation temperature.
Oil bearing fractures within the radius of investigation of resistivity tools cause anomalously high, apparent resistivity values in excess of the threshold resistivity.
The mechanisms of elevated resistivity response are conceived to be due to the averaging effect of 100% oil saturated fractures and partially saturated matrix, and the geometric confining effect of oil saturated fractures on the radius of investigation of resistivity tools.
Calculation of the triangulated resistivity anomaly (TRA) is an effective method of quantifying anomalously high resistivity. The TRA value is semi-quantitative and reflects the magnitude and proximity of an oil bearing fracture system to the well bore.
The threshold resistivity and TRA concepts indicate two probably fracture orientations in Silo field. A west-northwest/east-southeast trending system is the most pervasively distributed and likely consists of small, poorly interconnected fractures. More narrowly confined northwest-southeast trends are coincident with the most productive trend in the field and are interpreted to represent more open, widely communicated fracture trends.
Resistivity data and production data may not correlate due to the difference between data that indicate fracture communication with a well bore (production data) and data that indicate fracture proximity to a well bore (resistivity data) but not necessarily communication.

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