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PSSuccess! Using 
Seismic
Attributes and Horizontal
Drilling to Delineate and Exploit a Diagenetic Trap, Monterey Shale, San Joaquin
Valley, California*
By
Anne Grau1, Robert Sterling1, and Robert Kidney1
Search and Discovery Article #20011 (2003)
*Adapted for online presentation from poster presented at
AAPG’s annual convention, 2003, Salt Lake City, May, 2003. A companion article,
entitled “Delineation of a Diagenetic Trap Using P-Wave and Converted-Wave
Seismic Data in the Miocene McLure Shale, San Joaquin Basin, California,” is
Search and Discovery Article #20012 (2003).
1EOG Resources, Denver Colorado (anne [email protected])
The
Miocene Monterey Formation of California’s San Joaquin valley has long been
recognized as a prolific source rock and underdeveloped resource. In this case
study, the thick sequence of diatomaceous shales and hydrocarbon-rich sediments
of the Miocene form a subtle diagenetic trap. As these sediments are buried to
increasing depths, these siliceous shales convert from opal A to opal CT and
finally to quartz-phase “chert”, undergoing a significant change in porosity and
other rock properties during this transition. Seismic data and modeling have
been successfully utilized in the identification and mapping of these diagenetic
facies.
North Shafter and Rose Oil Fields produce from a porous, hydrocarbon-charged reservoir that formed as a result of silica diagenesis and favorable timing of kerogen maturation in these sediments. The reservoir consists of fractured, porosity-enhanced, oil-saturated quartz-phase rocks. A trap is formed by the updip, opal CT - phase rocks that have no hydrocarbon saturation and poor porosity characteristics.
The juxtaposition of these drastically different rock types is reflected by
seismic amplitude anomalies that were used to determine the extent and shape of
the fields. Horizontal drilling technology and strategic placement of wells have
been key in the viability of this program. Close to 60 horizontal wells have
been drilled in North Shafter and Rose oil fields since 1998, when the first
horizontal well was drilled.
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uStratigraphy, reservoir, & trap tReservoir character & trapping mechanisms uStructural & stratigraphic control
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uStratigraphy, reservoir, & trap tReservoir character & trapping mechanisms uStructural & stratigraphic control
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uStratigraphy, reservoir, & trap tReservoir character & trapping mechanisms uStructural & stratigraphic control
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uStratigraphy, reservoir, & trap tReservoir character & trapping mechanisms uStructural & stratigraphic control
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uStratigraphy, reservoir, & trap tReservoir character & trapping mechanisms uStructural & stratigraphic control
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Location, Stratigraphy, Reservoir, and Trap Figure Captions (1-7)
Right: Higher-magnification view of silty mudstone texture illustrates an abundance of discontinuous, horizontal microfractures and patches of microporosity (magenta). Close examination reveals that these microporous lenses (m) are not as clayey and are presumably composed of micro- or cryptocrystalline silica. Microfractures at the top of view are likely induced by dehydration. Also note abundance of pyrite and carbonaceous debris. Scale bar = 0.2 mm. Plane-polarized light. (100×).
North Shafter and Rose oil fields are in the San Joaquin Basin (Figure 1). The Miocene Monterey Formation, composed primarily of diatomaceous siliceous shales (Figures 2, 3, and 4), is the source and the reservoir in these fields.
The heterogeneous reservoir is of thinly-interbedded diatomaceous lithologies, represented by:
Reservoir Character and Trapping MechanismsReservoir is created by downdip conversion from Opal CT to Quartz Phase rocks coeval with hydrocarbon charging (Figures 5, 6, and 7). Formation of the trap accompanied creation of the reservoir, with the Opal CT phase rocks forming the updip seal.
Structural and Stratigraphic Controls on Reservoir Distribution Figure Captions (8-12)
Monclinal dip, approximately 4o to the southwest (Figures 8, 9, 10, and 11).
Heterogeneous reservoir (Figures 9, 10, 11, and 12), a function of: · Clay content · Depth and temperature
Figure Captions (13-14)
After
North Shafter Exploration Highlights Figure Captions (15-16)
Compared to the present, the area of Rose and North Shafter fields in 1995 was undeveloped (Figure 15). The highlights are:
Shown in Figure 16 is the production history of North Shafter and Rose fields, along production plot for two typical wells.
+ Diagenetic trap. + Unconventional: Siliceous Shale Reservoir.
+ + Exploitation via Horizontal Drilling. + “A Whole Lotta” Oil in Place. ___________________________________ = Success! Cumulative to date 4/1/03 Rose-- 1.2 million BO, 0.5 BCF North Shafter-- 5.1 million BO, 2.1 BCF Present Production (Peak Rate) Rose 900 BOD (2705 BOD) North Shafter 3600 BOD (5202 BOD)
Isaacs, C. M., 1981 Outline of Diagenesis in the Monterey Formation examined laterally along the Santa Barbara Coast, California: in Isaacs, C.M., ed., Guide to the Monterey Formation in the CaliforniaCoastal area, Ventura to San Luis Obispo, Pacific Section, AAPG, v. 52, p. 25-38.
EOG RESOURCES: Barbara Ganong, Chris Hanson, Paul Pendleton, Linda Hoagland, CB Lackey, Paul Connely Texaco/Chevron: Larry Drennan, Angela Boss, Bob Barree, Laurie Williams |
Figure
1. Location map of North Shafter and Rose Oil fields area.
Figure
2. Stratigraphic column, southern San Joaquin
Figure
3. Reservoir character of cored interval, from core analysis and
wireline-log analysis.
Figure
4. Photomicrographs of mudstone, Tulare 34-1, depth
interval--7655.5-7657.7 feet.
Figure
5. Diagrammatic description of “diagenetic trap.”
Figure
6. Conversion of opal to quartz with increasing temperature, affected by
clay content (modified from Isaacs, 1981).
Figure
7. Porosity vs. permeability from core analysis, according to lithology.
Figure
8. Structure map, Rose and North Shafter oil fields, on top of the
McLure Shale. 
Figure
9. Dip Cross Section, North Shafter Field. Stratigraphic Cross Section
1, with quartz phase downdip and opal CT phase updip. Inset: structural
cross-section.
Figure
10. Dip cross section, Rose Field.
Figure
11. Dip Cross Section, Transition Zone: Midas area.
Figure
12. Strike Cross Section, Rose and North Shafter Fields.
Figure
13. Map of North Shafter 
Figure
14. 
Figure
15. Status of North Shafter / Rose area; Left--circa 1995 (before
development of the fields; Right--present day.
Figure
16. Gross daily oil production at North Shafter and Rose fields and from
two wells. 