(Above image is 500x375. Original size is 640x480)
The Miri Field is geologically a very complex one and till today is not fully understood. The structural map
gives you an idea. There are three key outcrops in this section with a lot
of detail behind them (stratigraphical columns, detailed pictures). See
the historic Miri Well No.1 also known as the Grand Old Lady
EARLY HISTORY
Historical records of shallow, hand-dug oil "wells" in the Miri area date back to the 1880's but rudimentary production probably existed as early as the 11th century. In a Government report as early as 1882, Claude de Crespigny mentioned the occurrence of oil seepages at Miri. In 1902, Dr. Charles Hose, a well known naturalist, and de Crespigny's successor as Resident of Baram, prepared a map of surface oil indications in the Baram district.
INDUSTRIAL DEVELOPMENT
In 1909 the first Sarawak Oil Mining Lease was granted to the Anglo Saxon Petroleum Company, an offshoot of the recently formed Royal Dutch/Shell Group.
Geologist Dr. Joseph Theodor Erb evaluated the prospectivity of the various locations on Hose's map and identified the Miri Hill as the best prospect. The first geological map of the Miri Hill was finalised in 1910 (24th January 1910; revised 16th July 1910) and a well location was picked on top of the hill.
Miri-1 well was spudded in August 1910 and by December was producing 88 barrels/day of light crude from a sand at 995' depth, which would later be called the "No. 1 Sand".
By the end of 1971, the field produced no more than 675 barrels of oil per day (and more than ten times as much water) from 98 producers. In view of the declining production, the field was abandonment on the 20th of October 1972.
IMPORTANCE OF THE FIELD
The Miri Oil Field is the only hydrocarbon accumulation to have been commercially exploited onshore Sarawak. Some 624 wells have been drilled to drain the field. Development well locations were at very close spacing (300'-450')
The main oil accumulations were found within the first 2000', occasionally with a small gas cap. At 2000' depth, porosities of 20% were still encountered.
Typical production rates per well ranged from 10 - 150 barrels/day, when pumped. Peak production was achieved in 1929 (15,211 barrels/day). The total field recovery is estimated at some 80 million barrels of oil.
Geologist P. von Schumacher wrote on 7th February 1941 :"The cumulative gross production of the Miri field at the end of 1940 amounts to about 9.6 million tons, and it is expected that future production from existing wells will eventually bring this figure to about 11 million tons, thus placing Miri amongst the more prolific fields of the East Indies". Von Schumacher's prediction as to the ultimate production of the field proved remarkably accurate.
GEOLOGICAL DATA
The Miri structure consists of a complex NE-SW elongated anticline, bound to the east by a steep NW dipping fault, known as the Shell Hill Fault. The downthrown block of the Shell Hill Fault is dissected by a series of steep NW dipping subsidiary faults (W2-W6) and a series of flat SE dipping antithetic faults (E1-E12). Further to the east, and underneath the Shell Hill Fault, three NW dipping thrusts faults are present: the Canada Hill Thrust and the Inner and Outer Kawang Thrusts.
The Miri Field consists of some 78 separate fault blocks, of which 48 have been put on production; oil accumulations are found at different levels in the various blocks.
RESERVOIRS & SEALS
The Miri Formation consists of an alternation of coastal sandstones and marine shales of middle Miocene age (9-14 million years). The sandstones vary from very fine grained, laminated, cm-thick tidal deposits to medium grained, massive cross bedded or bioturbated shoreface and bar deposits; their lateral continuity is generally at the scale of the field. The poorest reservoirs consist of muddy, fine-grained storm sand deposits with poor lateral continuity.
Seven (7) main reservoir sandstones have been identified, of which 2, the "Upper C" and the "105" have produced nearly 60% of the total field production from 3 reservoir blocks alone. No production has been obtained from the top sands of the Upper Miri Formation.
Drilling results indicate a significant deterioration in sand thickness and quality to the north and west, suggesting the presence of a nearby shale-out for all sands.
Top seals consist of interbedded marine mudstones, characterised by key foraminiferal assemblages; such transgressive events can be recognised throughout the field and separate all main reservoir sandstones.
Clay smear along the Shell Hill Fault provide an effective lateral seal to the field. Although some oil shows can be observed to the east of this fault, the productive area clearly parallels the fault.
HYDROCARBON HABITAT
No distinct source rock interval has been identified in the Miri area. Mainly humic to mixed, landplant-derived organic material, dispersed within the early-mid Miocene mudstones and sandstones is thought to constitute the source rocks for the oil and gas.
DOM measurements indicate that the mature kitchen areas are located on the flanks and in the basinal areas adjacent to the structural hights.
Surface seeps and oil-impregnated sandstones are seen in many outcrops. Such indications were the original clues which led to the discovery of the field. The effects of water-drive mechanism can be observed during the rainy season, when abundant light oil films cover most of the water gutters, especially on the eastern part of Canada Hill.
TECHNOLOGY
Miri No. 1 and the early wells were drilled using the cable-tool method: repeated blows from the heavy drilling tools broke and pounded the rocks. At intervals, the spoil (mixed with water) was bailed out. In order to prevent hole collapse, steel pipes were inserted joint by joint to follow the tool. In 1925, the rotary drilling method was introduced and extensive coring was carried out.
The Schlumberger logging method was introduced in the late thirties and used sparingly. Electrical logs are available for only 34 of the 624 wells, and consist mostly of the early version of the Induction Log; only 2 wells have a modern suite of wireline logs.
Initial well correlations were based on the specific gravity of the oils. A major improvement in correlating reservoirs between wells, across the many faults was achieved in the mid thirties, when the micropaleontological method was introduced. Distinctive series of microfaunal assemblages (foraminifera) which followed each others in a fixed order were first identified in the Seria Field and later recognised in the Miri Field.
Seismic data were never acquired.
The geological model of the Miri Field has remained in broad lines the one proposed by von Schumacher in 1941.
