PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Fifth Annual Convention, June 1976

53

WRENCH FAULT TECTONICS AND ASPECTS OF HYDROCARBON ACCUMULATION IN JAVA

ABSTRACT Northwards movements of the Indian Ocean plate relative to the .4siar? plate are postulated to be closely associated with north-south lateral compression in Western Indonesia. This compression was. responsible for the forination of a meridional shear system in Java. This shear system has developed since the Late Cretaceous. In accordance with wrench fault tectonics concepts, an attempt has been made to prove the existence of an ordered wrench fault pattern in Jaxa applying th; reasonable values of Q! = 1 4 O p = 35 and? = 10 . All faults can be grouped into first-second- and third order wrench systems. Most of the folds *long to first order (primary) with some folds around Jakarta being ascribed to second order drag. In northeast Java almost all folds are of third order drag. Exposures of Pre-Terhary and Lower Tertiary sediments in the Lok Ulo drea and the shore line configuration of north Central Java are most probably controlled by first order wrench faults. Hydrocarbon aspects of the wrench fault system are well evidenced in the northeast Java basin, where the prolific hydrocarbon bearing structures known since the last century can be grouped in en echelon folds closely related to the third order left lateral wrench. A suite of westeast trending faults interpreted from gravity data are most probably related to the existence of the west-east trending arc trench system in Java, as a result of continuing northwards movemenis of the Indian Ocean plate during the Oligocene.

INTRODUCTION Twenty years have passed, since Moody and Hill (1 956) introduced their wrench-fault tectonics concept. According to this concept, it is believed that large-scale wrench faults occurred in the earths crust as a result of two m a o r compressional strain systems either equatorially or meridionany oriented. Move- ments on these faults will produce lower-order folds, thrust faults, and wrench faults in a characteristic assemblage. Following the Moody and Hill (1956) defini-

B. SITUMORANG * SlSWOYO *

ENDANG THAJIB * F. PALTRINIERI *

tion, the term wrench fault used in this paper is an equivalent of strike - slip fault, transcurrent fault, or lateral fault. The importance of strike-sIip faults in con- trolling sedimentation and structural develop- ment has been broadly discussed by Rod (1956, 1960, 1962). From a purely tectonic beginning, the wrench-fault tectonics concept was later close- ly related and applied to petroleum explor- ation. Corey (1962) discussed the effects of lateral faulting on petroleum exploration. He is of the opinion that recognition And proper interpretation of small or large lateral fault systems i s very helpful in locating petroleum reservoirs and traps. Petroleum exploration aspects of wrench-fault tectonics have also been discussed by Moody (1 973) and illustrated by examples from some oil provinces, e.g. Alaska, Gulf of Guinea, etc. The relationship of the wrench system with prolific en echelon folding has been shown by Wilcox et.al. (19731, while Harding (1973, 1974) stressed the importance of wrenching deformation, and also the association between traps and wrench systems. - Some major transcurrent faults are known to occur in Indonesia, for example the Semangko fault of Sumatra, the Fossa Sarasina, and Gorontalo faults of Sulawesi, and the Sorong fault of Irian Jaya (Katili, 1969). So far there has been no real attempt known to the authors, to explain the occurrences of these lateral fault systems in terms of the wrench- fault tectonics hypothesis. Many geologists, working in the region, have tried to obtain a better understanding of the geological framework by applying the recently developed plate tectonics concept.

*) LEMIGAS (Indonesian Petroleum Institute), Exploration Division - Jakarta

IPA, 2006 - 5th Annual Convention Proceedings, 1976

54

The authors believe that there is a relation- ship, not yet fully understandable, between these two hypotheses. It is possible that the lateral compression, indicated by Moody and Hill (1956) as the original cause of the crustal shear system, is dso responsible for the motions of the crustal plates. Alternati- vely, the plate motion itself could be considered the only source for the two primary compressional forces which resulting in the formation of the wrench-fault system. Nevertheless, it is not the intention of the writers to deepen this problem; the purpose of our paper is to analyse the structural style of a specific area and try to explain it in terms of the wrench-fault tectonics concept. Java island (fig.1) was choosen as an example, since many faults mapped in Java show strike slip movement (Ranneft, 1972, Lemigas/ Beicip, 1969). Moreover, this island has an unique position in Western Indonesia. It lies approximately at the intersection between two major crustal shears, trending Northwest - Southeast (NW-SE) in Sumatra and Northeast - Southwest (NE-SW) in Kalimantan as interpreted by Moody (1966), (fig. 2). It seems, that the fault systems trending Northwest - Southeast (NW-SE) in Sumatra and Northeast - Southwest (NE-SW) in the East Java Sea and Eastern Kalirnantan, which are known to have been in existence since at least the late Cretaceous, have the same trend (direction) as those of the two major crustal shears mentioned above. We should emphasize also that it is not our intention to present a solution to the tectonic framework of Java.

FAULT PATTERN AND INTERPRETATION According to Moody and Hill (19561, the wrench-fault tectonics theory supposes that lateral compression, active since the formation of the earths crust, caused some deformation of the crust along complementary wrench fault zones. The lateral compression caused two main shear patterns, named, according to their Stress direction, meridional and equato- rial shear patterns. It seems that the patterns could exist independently but in many cases Moody (1973) stated that the meridional shear system has been superimposed on a somewhat older equatorial shear pattern.

Figure 3 illustrates a plan of meridional shear pattern created by North - South compres- sional stresses. The results will be the formation of first -, second -, third -, and higher order shears, however, the continuing lateral compression will result in the repetition of shear directions in the third order shears. Katili (1975) postulated that in the middle Cretaceous an active subduction zone and magmatic arc occurred in Western Indonesia as a result of northward movements of the Indian Ocean plate relative to southward movements of the Asian plate (fig.4). The authors were greatly impressed by the direction of these motions and postulated that they could be closely associated with a North - South lateral compression (stress system) which could produce a wrench-fault system as presented in figure 3. Thus, it was decided to apply this model in analysing the structural style of Java, especially regarding the fault and fold system of the island. All detectable data related to the wrench faults in Java were compiled and plotted (fig.5). In addition, some major faults inter- preted from gravity data were also included (Untung and Hasegawa, 1975). Keeping in mind Moodys quotation that a structural pattern can be built up if only the dip, strike and relative motions of one fault are known accurately, or if the strike and asymmetry of one anticline are known accurately, and if reasonable values for a, 0 and 7 are used, a close observation on fig.5 has been made showing the existence of a first order wrench, trending Northeast - Southwest (NE-SW). The left lateral fault passes near the town of Semarang in Central Java and extends northeasterly into the offshore area. Although it is derived from gravity data, it is very important to note that the southwestern onshore portion of this fault has been recognized as a left lateral fault by Sukendar (1974, fig.9). I t is supposed to be a continuation of the lateral fault system active in the East Java Sea Basin since (at least) Late Cretaceous (Sukendar, 1974, fig.8). Further- more a set of right lateral faults was recognized in the Sunda Straits (Ranneft, 1972). Combining all these data with the fold-trend

map (fig.6), and taking into account that two fault systems trending Northwest - Southeast (NW-SE) and Northeast - Southwest intersect south of Java, a mefidional wrench-fault system caused by a North-South lateral compression can be constructed based on the values of a = 14 , fl = 35 , and7 TM 10 , (fig.7). It is clear that most folds in West and Central Java belong to the first order and that folds around Jakarta belong to the second order systems, while in North East Java all the folds are of third order drag. The left lateral Lembang fault (Tjia, 1968 in Katili, 1969, rigA) was recognized as a second order left lateral, and a set of faults in the Sunda Straits belong to the second order right lateral wrench. The faults interpreted from gravity data (Untung and Hasegawa, 1975), which trend Northwest - Southeast (NW-SE) from East of Jakarta on the North coast down to Cilacap on the South Coast in West Java, and a Northwest - Southeast (NW-SE) trending fault in East Java, probably belong to the complementary first-order wrench set; while the one trending Northeast - Southwest (NE-SW) appears to' be a primary first-order wrench (fig.5), at least during the early stage of its formation. Since we do not have any proof regarding the relative motions of all the West-East (W-E) trending faults interpreted from gravity data, we suspect that this group was probably related to the existence of the West - East arc-trench system during ? late Oligocene time, as a result of continuous northward movements o f .the .Indian Ocean plate. Turning back to fig.7, i t seems that the shore line configuration in the north of Central Java was strongly controlled by the horizontal movements .of the first-order wrench fault system. The same ease could be recognized for the coast line in the westernmost part of Java, which was strongly afleeted by the second fight lateral wrench.

By Moody's interpretation (1973, fig:5) there is a tendency for relative vertical movements to occur in the block bounded by first-order wrench faults, due to vertical relief of lateral pressure. This fits very well with a gravity inter- pretation, which indicates that the internal

55

block between the first-order wrench faults (block I) is lifted up relative to the external blocks (blocks lI and III), (fig.7). The authors postulate that the most extensive outcrops of the Pretertiary and lower Tertiary sediments in Central Java (Lok Ulo area) are closely related to the ulflifting of block I.

HYDROCARBON ASPECTS

Moody (1973) was of the opinion that prospective petroleum basins could be deline- ated, by analysing the regional fault pattern in accordance with the wrench-fault tectonics hypothesis. Prediction of tectonic style and the history of basin formation can be deduced also from this analysis. Furthermore, he stressed, that "drag folds associated with wrench faults provide the most obvious traps for oil in an area of wrench-fault tectonics". These drag folds are usually arranged in "en echelon" system. In 1969, a regional geological and geophysical study of the North East Java basin has been carried out by Lemigas/Beicip. It was interpreted that the basin was dissected by a set of Northeast - Southwest (NE-SW) left lateral faults, and all the folds were found to be disposed in an "en eehel9n" pattern (fig.9). Without mentioning .any relation to Moody and Hill's concept, the report stressed the existence of a t ight relationship between faulting crustal blocks and the "en echelon" folding systein formed in the sedimentary cover (fig. 10). This interpretation was in accordance with -Moody and Hill's opinion (1956) that "systems of small "en echelon" folds or anticlines appear to be surface indications of some deep seated wrenches;'. Our examination of the North East Java basin structural style eonfirrns the existence of this "en echelon" folding pattern strictly related to the wrench- fault tectonics. In fact the Northeast - Southwest (NE-SW) trending left lateral wrench faults belong to the third order left lateral wrench, while the "en echelon" system itself could be ascribed in to the third order d~ag. Since some 150 million baiTels of crude have been produced f rom these structures during the last 70 years, there is]ust i f ieat ion for the suggestion emphasized by Wilcox et.al.

56

(1973), Moody (1973) and Harding (1974), that such a fold pattern associated with crustal shear tectonics is a favourable structure. for oil accumulation. Distribution of hydro- carbons, seems also to have been affected by the fragmentation of Java, caused by the fint-order wrench fault. Java island has been fragmented into three blocks, namely blocks I , I1 and I11 respectively, where block I was vertically uplifted With respect to blocks I1 and I11 (fig.7). Consequently, block I could potentially under- go more intensive ermional processes and become the source of clastics, which were deposited in the surrounding down blocks (I1 and 111). A vast deposition then would have taken place in these blocks, resulting in a more favourable condition for hydrocarbon genesis and accumulation. In support of this thesis it can be noted that all the known commercial oil-fields in Java were situated either in block I1 or 111, and only a small previously exploited oil-field (Cipluk oil-field, near Semarang) was found in block I (fig.8).

5. It appears that Java has been frag- mented into three main +locks by first order wrench faults. Distribubon of hydrocarbons seems to be affected by the fragmentation of the island.

6. Most of the prolific hydrocarbon bear- ing structures, in the North East Java basin, are strictly related to the en echelon folds.

ACKNOWLEDGEMENTS The authors would like to thank the manage- ment of LEMIGAS for perniission to publish this paper. We are greatly indebted to DR. J.D. MOODY formerly Senior Vice-president, Exploration and Producing, Mobil Oil Corporation, who reviewed and corrected the manuscript. We also are indebted to DR. SUKENDAR ASI- KIN of ITB, and MR. A. PULUNGGONO of PERTAMINA, for fruitful discussions we have had during the completion of this paper. Finally, our sincere thanks are extended to MR. C.D. BOYCE who helped us in linguistic rectifications of the manuscript.

CONCLUSIONS REFERENCES From the foregoing discussion, some conclu- sions can be drawn as follows :

1. In accordance with the wrench-fault tectonics concept, a meridional shear system has been interpreted to exist in Java, as a result of a North - South lateral compression closely related to a northward movement of the Indian Ocean plate relative to the Asian plate.

2. First -, second -, and third order wrenches have been recognized in Java. Most folds belong to the primary folding system, while only some folds around Jakarta are ascribed to the second order drag.

3. Third order drag mainly located in North East Java, is related to the third order left lateral wrench. These third order drag folds are usually disposed in an en echelon pattern.

4. The Coast line configuration in the north of Central Java, and the extensive outcrops of the Pretertiary and lower Tertiary sediments in Lok Ulo atea, tend to be strongly controlled by the first-order wrench fault movements.

ALBERDING, H., 1957 : Application of princip- les of wrench-fault tectonics of Moody and Hill to northern South America, Geol.Soc. America Bull., vo1.68, pp.785 - 790.

COREY, W.H., 1962 : Effects of lateral faulting on oil exploration, Am.Assoc.Petroleum Geo- logists Bull, vo1.46, No.12, pp.2199 - 2212.

The Newport - Ingie- wood trend - an example of wrenching style of deformation. Am.Assoc.Petro1eum Geo-

, 1974 : Petroleum traps as- sociated with wrench faults, Am.Assoc.Petro- leum Geologists Bull., vo1.58, NO., pp.1290

KATILI, J.A., 1969 : Large transcurrent faults in South East Asia, with special reference to Indonesia, Bull.NIGM., Bandung, vo1.2, No.3, pp.1 - 20.

- ,1973 Geochronology of West Indonesia and its implication on plate tecto- nics, Tectonophysics, vo1.19, pp.195 - 212.

- -_________ , 1975 : Volcanism and plate tectonics in the Indonesian Island Arcs, Tectonophysics vo1.26, pp.165 - 188.

HARDING, T.P., 1973 :

logists Bull., V01.57, No.1, pp.97 - 116.

- 1304.

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LEMIGAS/BEICIP, 1969 : North East Java Ba- sin, Unpublished Report, Lemigas, Jakarta.

MOODY, J.D., AND HILL, M.J., 1956 : Wrench

ROD, E., 1956 : Strike-slip faults of northern Ve- nezuela, Am.Assoc Petroleum Geologists Bull., ~01.40,, No.3, pp.457 - 416.

fault tectonics, Geol. Soc. America Bull., vo1.67, pp.1207 - 1246.

, 1964 : Moody and Hill system of wrench-fault tectonics. Reply : Am.Assoc.Petroleum Geologists Bull., vo1.48,

MOODY, J.D., 1966 : Crustal shear pattern and orogenesis, Tectonophysics, ~01.3, pp.479 - 522.

, 1973 : Petroleum exploration aspects of wrench-fault tectonics, Am.Assoc. Petroleum Geologists Bull., ~01.57, No.3, pp.449 - 476.

Moody and Hill system of wrench fault tectonics, discussion : Am.As- soc.Petroleum Geologists Bull., vol.48, No.1,

The effect of con- tinental drift on the petroleum geology of Western Indonesia, Austral.Petr.Explor.Ass., v01.12, Part 2. pp.55 - 63.

No.1, pp.112 - 122.

PRUCHA, J.J., 1974 :

pp.106 - 111.

RANNEFT, T.S.M., 1972 :

, 1960 : Strike-slip fault of continental ,importance in Bolivia, Am.Assoc. Petroleum Geologists Bull., vo1.44, No.1,

___---_ _-- , 1962 : Fault pattern, north- west comer of Sahara shield, Am.Assoc. Petroleum Geologists Bull., ~01.46, No.4,

pp.107 - 108.

pp.529 - 534. SUKENDAR, A., 1974 : The geologic evolu-

tion of Central Java and vicinity in the light of the newglobal tectonics, Disertasi DR., Institut Teknologi Bandung.

UNTUNG, M., AND HASEGAWA, M., 1975 : Penyuwnan dan pengolahan data beserta penafsiran Peta Gaya Berat Indonesia, with abstract in English, Journ. of the Assoc. Indonesian Geologists, pp.11 - 17.

WILCOX, R.E., HARDING, T.P., and SEELY, D.R., 1973 : Basic wrench tectonics: Am. Assoc.Petroleum Geologists Bull., voL57, No.1, pp.14 - 96.

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