Marulk

Marulk was discovered in 1992 but production wasn’t started until 2012. The field is

developed with a subsea template tied-back to the Norne field 25km to the north-east. It is located on the Dønna Terrace. It is an elongated structure with SW-NE trending faults defining its north-west border. The structure is upthrown, juxtapositioning the main reservoir, the Lysing Formation, to younger strata of the Shetland Group across the fault.

The primary objective of the exploration wells 6507/2-1 and 6507/2-2 were actually in the Jurassic section; formations in the Fangst and Båt Group. The first well, 6507/2-1 (1986), was dry with shows in the primary objective Jurassic section, but also proved the presence of Cretaceous Lysing and intra Lange Formation sandstones of the Cromer Knoll Group. The Cretaceous sands were also dry with shows. As the first well left considerable potential volumes up-dip, well 6507/2-2 was spudded in October 1991. The primary objective was still

Figure 5.2: Interpreted top Cromer Knoll Group surface showing the structure and surrounding area.

Approximate field outline based on depth contours and ODT/WUT.

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the Jurassic reservoirs of Fangst and Båt Groups. The secondary objectives were the

sandstones in the Lysing and Lange Formations. The Jurassic section was dry, but both intra Lange and Lysing sandstones were gas filled. Later, in 2008 well 6507/2-4 was drilled to apprise the discovery and prove the commerciality of the Lysing discovery. Intra Lange sandstones was the secondary objective. The Lysing formation sandstone was penetrated at 2832m MD and was gas filled down to its base, giving a GDT (gas down to) at 2852.5m MD.

The upper sandstone unit in the Lange formation was composed of sand layers interbedded with claystone. It was water wet. A deeper, 3 meters thick sand body at 3378.5m MD was oil filled.

Figure 5.3 shows a typical cross section of the structure, presenting the general geometry.

The Lange Formation has a very homogenous appearance in the seismic, making it very

Figure 5.3: Interpreted cross section of Marulk structure. See figure 5.2 for location of the seismic line

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difficult to pick the tops of the Intra Lange sands. The thickness of the oil filled Lange-sandstone layer from well 6507/2-4 is well below the vertical resolution of the seismic.

The main focus of this subchapter is the gas accumulation in Lysing Formation. Table 5.1 summarizes data from the wells. The formation is thinnest in the most southern well, and thickest in the northern well. No gas-water contact was encountered. One well was dry with shows and in the other two, Lysing was gas filled down to its base.

Figure 5.4 plots the pressures from each well in Lysing, and compares it to pressure measurements in the upper intra Lange Sandstone unit. It is reasonable to assume lateral

Table 5.1: Table containing information on the Lysing Formation from different wells on the Marulk structure. TVD = TVD MSL (True vertical depth from mean sea level)

Well

Figure 5.4: Formation pressure data from Lysing Formation and the upper intra Lange sandstone in wells 6507/2-2 and 6507/2-4. The pressures from well 6507/2-4 and 6507/2-5 falls on roughly the same gradient

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pressure communication in Lysing formation. There is a pressure barrier between Lysing Formation and the upper Intra Lange Fm.

Lysing Formation is clearly visible in the seismic where it is present, and of sufficient

thickness. It’s usually represented by a sharp decrease in acoustic impedance, giving a clear blue through in the seismic. When Lysing is not present, the top of the Cromer Knoll Group corresponds to top Lange Formation. Top Cromer Knoll group is generally a red peak (increase in acoustic impedance) in locations where Lysing Formation’s seismic signature is absent. As Lysing Formation gets thinner, the reflector changes its appearance.

Figure 5.5 shows a seismic inline through the Marulk structure. The appearance of the top Cromer Knoll reflector is varying across the structure. Note the typical seismic signature of top Cromer Knoll Group when Lysing Formation is present to the right. The signature dims as you go left and approach the fault. Top Lange is very close to top Lysing in the well location.

The thickness of Lysing Formation is 13,5m in the well.

Figure 5.5: Seismic inline through well 6507/2-2. Including formation tops from Petrel, fault interpretations and general observations.

(top Cromer Knoll Gp.)

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In order to map the extent and amplitude of the Lysing deposits, an RMS interval average surface attribute was extracted at +/-20ms from the interpreted top Cromer Knoll Group.

Amplitudes are varying laterally in the Marulk area. The amplitudes in the proximity of well 1, where Lysing Formation is only 5m thick, are dim. In the proximity of well 6507/2-4, where the formation is thicker, the amplitudes are bright. The weak amplitudes closer to the fault, seen in figure 5.5, is seen clearly on the RMS amplitude surface. The brightest areas of the amplitude map is usually where the sand is believed to contain gas, but there are bright areas outside the accumulations as well, so the map is better used for interpreted sand distribution than gas content.

Figure 5.6: RMS amplitude extracted on top Cromer Knoll surface. High amplitudes are bright yellow.

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As seen in figure 5.7, the location of an exact spill point is uncertain because of the flat topography. By lowering the horizontal plane with just 4ms (minimum step for the horizontal plane), the structure goes from not being filled to structural spill, to seemingly spilling at more than one location. The most likely spill point is located in the area encircled in red, but the uncertainties in the interpretation and depth-time correlation is greater than 4ms.

Assuming the spillpoint is at 2748ms, using the checkshots for the two closest wells, 6507/2-4 and 6507/3-9 S, gives a depth of 2872 m TVDSS (+/-25 m)

Figure 5.7: Spill point interpretation. Horizontal plane at TWT=2744ms (left) and at 2748ms (right). Most probable spillpoint in red, alternatives in orange.

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