PROCEEDINGS JOINT CONVENTION BALI 2007
The 32^nd HAGI, The 36^th IAGI, and The 29^th IATMI Annual Conference and Exhibition

FOLD GENERATE FAULT STUDY IN SEMAYANG AREA,
EAST KALIMANTAN

Fuad Ahmadin Nasution (¹), Armein Suleiman (^1,2), Andang Bachtiar (³)

(1) GDA Consultant
(2) Magister Program, Department of Geology, Institute of Technology Bandung
(3) Exploration Think Tank Indonesia (ETTI)

ABSTRACT

This study is referring to Mitra’s theory and models, 2003 which is study of fold development which generate secondary fault known as fold accommodation fault. Fold accommodation fault are secondary faults that accommodate strain variations related to structural and stratigraphic position during fold evolution. Four main fault accommodation faults are commonly fault found. Out of syncline and into anticline thrusts form primarily because an increase in bed curvature within fault. Forelimb and backlimb thrusts form by a variety mechanisms. Forelimb space accommodation thrusts are low displacement thrust that resolves strain discontinuities resulting from increased curvature in fold cores. Forelimb shear thrusts form in late stage of folding because of rotation and layer-parallel extension on the steep forelimb of folds. Most of backlimb thrust is originated as out of syncline thrusts.

This study intended to analyze the fold generate fault based on application of fold accommodation fault theory and its type which is found in study area. The study is focused on syncline and anticline in the Semayang area where is situated in Kutai basin, East Kalimantan. An understanding of kinematic evolution and slip distribution of fault in Semayang area is important to developing accurate interpretations of structures beside the models of structures constructing balanced cross sections and interpreting of geometry hydrocarbon trap forming structures. On the other hand, slip distribution of these kinds of fault is important to describe fault sealing capacity of related fault.

The structural formed due to tectonic event in Kalimantan since Middle Miocene (15-1 0Ma) which formed northeast-southwest trending Mahakam foldbelt. It is believed there some deltas were developed during that times formed huge sedimentary accumulation in the basin which were gave encumbering of sedimentation that triggering basin subsidence and structural forming. Skin-thin tectonic deformation also contribute to forming structures with developing of decollement zone on massive pro-delta deposits which were forced by tectonic sliding of the highs on the edge of Kutai basin to start resulting the anticlinorium.

About seven seismic lines interpret to define the fold accommodation fault pattern. The interpretation results was found five of some common types of faults accommodation faults are; out of syncline thrust propagating on the gently dipping limb, limb wedge thrusts (Figure-1), hinge wedge thrusts, forelimb and backlimb thrusts.

In conjuction to tectonic, some publication has been informed reconstruction of tectonic evolution in Kutai Basin. Early publication interpret that the tectonic is forming due to reactivated graben at second due to collision on North West Borneo Margin and East Sulawesi subduction in Mid Miocene (McClay, 2000) and Late of Early Miocne (Chamber et al, 1995). On the other hand, another hypothesis interprets that uplift and tectonic event is not relates to Sulawesi Collision (Calvert and Hall, 2003). Refer to these hypotheses, source of fold accommodation fault requires more discussion to summarize which one of the tectonic impact influence to form fold accommodation fault. Current analysis has been generated some

PROCEEDINGS JOINT CONVENTION BALI 2007
The 32^nd HAGI, The 36^th IAGI, and The 29^th IATMI Annual Conference and Exhibition

hypothesis that secondary fault is forming due to collisions of North West Borneo margin which is interact with prodelta shales. This hypothesis is generated based on occurrence of mud volcano adjacent study area and the presence of regional overpressure which refer of some publication (Bates, 1996).

The implication of this study is to review hydrocarbon potential in Semayang Area. The occurence some oil field and also oil and gas seepage adjacent area is informed that petroleum system occur inside study area. Three petroleum system has been defined in Kutai Basin are; Upper Neogene whereas source rock was generated from Midlle to Miocene Balikpapan Group and Late Miocene to Pliocene Kampung Baru Formation, the migration process is continuous and beleived to be started in Late Miocene to Pliocene time, and still most probably being occured at present day, Lower Neogene Petroleum System whereas source rock was generated from Miocene coals and carbonaceous shales of deltaic associated sediments. Time of migration was probably conducted during Middle Miocene to Late Miocene. Paleogene times whereas source rock is marine shale of Middle to Late Atan Formation. Time of migration was unknown (Pertamina BPPKA, 1997, Paterson et al, 1997). Clastic sediment and Reefal Carbonate are well known as potential reservoir in Kutai Basin. Turbidite reservoir in Pulau Balang Formation and Deltaic Sediment in Kampung Baru and Balikpapan Formation has been established contained hydrocarbon. Intraformational seal is interpreted to be potential seal in Semayang Area. The result of study interpret that hinge wedge thrust is potential trap; (Figure-2) the results of analysis which is incorporating with theory indicate that the geometry or reservoir is thick due to increased thickness on compression stage beside the sealing fault could be occurred during fold evolution.