3D Modeling of Discrete Fracture Networks in a Complex Fold-and-Thrust Belt, in the Northern Portion of the Eastern Cordillera Foothills, Colombia: Implications for Petroleum Exploration and Production Andres Acevedo (1), Freddy Corredor (2), David Richards (3), John Jairo Aristizabal (4) (1) Departamento de Geociencias, Universidad Nacional de Colombia, Bogota, phone: 571-483-6775, jandresacevedo@yahoo.es, (2) Dept. of Earth & Planetary Sciences, Harvard University, 20 Oxford St, Cambridge, MA 02138, (3) Midland Valley Exploration Inc, Golden, CO, (4) Ecopetrol S. A, Bogotá D. C, N/N, Colombia.
Study area (Figure No.2)
Tertiary
1'285.000 1'280.000 1'270.000
1'270.000
Gibraltar 1
Paleozoic - Pre-Cambrian
74 N º 7 S
ea
7
Pacific Ocean
COLOMBIA
N º 4 7 W º 5
7 N º 5
North 0
20
100 Km
Modified from Corredor, 2004
74
ºW
Indio Anticline
Cobaria monoclinal
Gibraltar structure
W º 2
860.000
W º 3
3
870.000
880.000
Ecopetrol S.A 2002 890.000
Figure No 3. Three-dimensional perspective view (see 3D cube) of the south-eastern face of the thrust imbricate system, showing the location of the Gibraltar structure (actual economic interesting structure), Cobaria monoclinal and the Indio anticline. It was interpreted as a thin-skinned fold and thrust belt. The thrust system consist of a serial thrust sheets with antiformal stack and arranged as a Break-forward imbricate structure. These multiple thrusts joining at the same detachment level and has a tectonic transport towards the East-Northeast like most of the major reverse fault in the northern part of the Eastern Cordillera. La Luna Formation serves as regional detachment in this model as well overall the Cordillera. 2D Move was used to validate all the structural section and after that export 2D data in a 3D system, in this case was 3D Move. The algorithm Tesselation was used to construct individual horizons one at a time, either stratigraphic units or faults.
Fault Stratigraphic Units:
Tertiary
e b ib r Ca
an
N º 5
1'260.000
ºW
1'260.000
Jurassic-Triassic
1'265.000
N º 6
Early Cretaceous
1'2650.000
Late Cretaceous
Figure N. 2. Geologic map of the eastern margin of the Eastern Cordillera Colombia, showing the location of the seismic profiles (violet lines) and numbering the main structures. Folds: A: Rio Negro Anticline, B: Rio Margua Anticline, C: Indio Anticline, D: El Silencio Anticline, E: El Deseo Syncline and F: Cobaria Monoclinal. Faults: 1: Cubugon fault, 2: La Frina fault, 3: San Jose fault, 4: Samore fault, 5: Banderas fault, 6: la China fault. The structures on the west side of the map strike NW-SE like Rio Negro and Indio Anticlines, La Frina and Samore faults, while the structures located on the east side strike NE-SW like Rio Margua and El Deseo folds, and La China fault. Stratigraphic Units: La Luna Formation presents beds and levels of mudstone and black chert concretion interbedded with black shale. It represents the main generating oil encountered in Gibraltar. Colon-Mitu Juan Formation was not penetrated by Gibraltar-1 but could be a potentially interesting reservoir unit. It is composed for a black shale massive succession, limestones locally interbedded with fine beds of mudstone and siderite. Sediments of the Barco Formation were derived from the Guyana Shield as fine quartzarenite with shale intercalation. Cuervos formation represents a transgressive unit and serves as a local seal to the underlying Barco Formation. The Mirador Formation is the principal reservoir and main exploration target of the Llanos Foothills and Llanos basin. It is predominantly a fluvial fine-grained quartzarenite, well sorted and cemented, and it is intensely fractured and this contributes to its abnormally high permeability values. Above the Mirador Formation is the thick Oligocene Carbonera Formation; this unit is predominantly a shale unit intercalated with muddy sandstone. The Carbonera is a proven excellent seal unit. The blue circle shows the Gibraltar location, the only one wildcat in the area with several side tracks. This borehole reached the Barco Formation, the reservoir quality was proven positively, and evidence of hydrocarbons was detected and measured. The Mirador formation, was formally tested and it produced 600 barrels per day of very high quality oil, 57 API, and 44 million cubic feet of rich gas per day (Villamil et al, 2003)
Carbonera 1 Fm. Carbonera 3 Fm. Carbonera 5 Fm. Mirador Fm. Cuervos Fm. Barco Fm.
K
73
1'275.000
K
W º 2 N º 7
ºW
890.000
1'275.000
7
2
880.000
1'280.000
Figure No. 1 Generalized geologic map of the Eastern Cordillera Colombia showing the main tectonic features and stratigraphic units (Corredor, 2004). The Eastern Cordillera of Colombia represent the leading edge of deformation in the northern Andes, it is a fine example of a complex compression region. This leading NE trending fold belt is bounded to the east by the Llanos foreland basin and to the west by the Magdalena Basin, by both east and westverging high angle and low angle reverse faults (Colleta et al., 1990, Dengo & Covey, 1993). In this northern portion of this fold belt, the structures have a NW-SE trend, which contrast with the overall NE-SW trend that characterizes most of the Eastern Cordillera. And particularly it changes again, in Venezuelan territory, to NE-SW. Folded and faulted sedimentary rocks dominate the panorama geologic, with ages ranging from Paleozoic to recent. The study area (violet square) represents a new frontier exploration area in Colombia, nowadays operated by Ecopetrol S.A.
1
Stratigraphic Units: Carbonera 1 Fm. Carbonera 2 Fm. Carbonera 3 Fm. Carbonera 4 Fm. Carbonera 5 Fm. Carbonera 6 Fm. Mirador Fm. Cuervos Fm. Barco Fm. Colo-Mitu Juan Fm. La Luna Fm.
870.000
1'285.000
ABSTRACT A recent discovery in the northern culmination of the Eastern Cordillera foothills, Colombia tested hydrocarbons from highly fractured Cretaceous-Tertiary reservoirs on eastward-verging, imbricated thrust sheets, above ramps which cut through the Cretaceous La Luna Formation. From a relatively image data, we constructed a 3D model of this fold belt that was sequentially restored and forward modeled using different directions of shortening based on two models of regional basin development. Model 1 proposes a single episode of NW-SE directed shortening, and Model 2 proposes two different events of deformation with an initial episode of SW-NE oriented shortening, followed by an episode of NW-SE oriented shortening. For each restoration we model the 3D discrete fracture network, including joints parallel to fold axis and conjugate sets of fractures, based on the dilatation of surfaces between the unfold and fold stages, and maximum stress direction respectively, which is key for new exploration and production efforts in this region. In order to test the two kinematic hypotheses and implicitly the most appropriate simulated fracture network, we compare observed fracture data from a satellite image, surface geologic mapping and two borehole image logs with the modeled fractures. The two episodes model yielded three main fracture patterns in common with observed data (N5W, N29E and N81E). These results suggest that the northern part of Eastern Cordillera may have formed by two shortening episodes of different orientation during the uplift of this thrust belt.
Tertiary
860.000
Colo-Mitu Juan Fm.
N
Acevedo, 2005