Visualizing Hydrocarbon Migration Pathways Associated with the Ringhorne Oil Field, Norway: An Integrated Approach

Norway’s Ringhorne Field is a faulted anticline which produces oil from Triassic (Statfjord) and Paleocene (Hermod) sands. It is located on the Utsira High. Geochemical studies of the produced oil indicate the oil is generated from mature Upper Jurassic marine shales in the adjacent Viking Graben. However, it has not been clear how oil migrated into the Triassic reservoirs and charged the overlying Paleocene reservoirs. Gas chimney detection using a proven neural network technique was used to detect the vertical hydrocarbon migration pathways on normally processed seismic data. The processing results were then validated using a set of criteria to determine if they represented true hydrocarbon migration rather than seismic artifacts. The chimney processing results using this traditional (shallow) neural network was compared with convolutional neural network (deep learning) results and geo-mechanical modeling on key lines. Key reservoirs were delineated using a stochastic (elastic) inversion approach. Reliable chimneys were then visualized in the vicinity of the producing reservoirs. The results showed pathways by which the Triassic fluvial sands received charge, and how these reservoirs had flank leakage to provide charge to shallower Paleocene reservoirs. This approach has now been used over hundreds of fields and dry holes in the Norwegian North Sea and worldwide as analogs to assess hydrocarbon charge and top seal risk predrill.

Coupled Basin and Hydro-Mechanical Modeling of Gas Chimney Formation: The SW Barents Sea

Gas chimneys are one of the most intriguing manifestations of the focused fluid flows in sedimentary basins. To predict natural and human-induced fluid leakage, it is essential to understand the mechanism of how fluid flow localizes into conductive chimneys and the chimney dynamics. This work predicts conditions and parameters for chimney formation in two fields in the SW Barents Sea, the Tornerose field and the Snøhvit field in the Hammerfest Basin. The work is based on two types of models, basin modeling and hydro-mechanical modeling of chimney formation. Multi-layer basin models were used to produce the initial conditions for the hydro-mechanical modeling of the relatively fast chimneys propagation process. Using hydro-mechanical models, we determined the thermal, structural, and petrophysical features of the gas chimney formation for the Tornerose field and the Snøhvit field. Our hydro-mechanical model treats the propagation of chimneys through lithological boundaries with strong contrasts. The model reproduces chimneys identified by seismic imaging without pre-defining their locations or geometry. The chimney locations were determined by the steepness of the interface between the reservoir and the caprock, the reservoir thickness, and the compaction length of the strata. We demonstrate that chimneys are highly-permeable leakage pathways. The width and propagation speed of a single chimney strongly depends on the viscosity and permeability of the rock. For the chimneys of the Snøhvit field, the predicted time of formation is about 13 to 40 years for an about 2 km high chimney.

Gas Migration Mechanisms and Their Effects on Caprock Seal Capacity: A Case Study in Depleted Gas Fields in the Sarawak Basin, Offshore Malaysia

This paper focuses on the gas characteristics in caprock interval and the gas migration mechanisms from the carbonate reservoir into the caprock and its effects on caprock seal capacity. The workflow mainly includes three methods:(1) Gas geochemistry analysis from the GWD (Gas While Drilling) data to understand the gas composition, their distribution and mechanism for gas migration; (2) Petrophysical analysis to understand the rock types, petrophysical properties and the pore-throat system; and (3) Pore pressure prediction to understand the pressure sealing capacity of the caprock. Integrating the results from these three aspects, the sealing capacity can be evaluated by capillary pressure sealing, pore pressure sealing and the effects on the sealing efficiency for CO2. There are two gas migration mechanisms in the area: gas diffusion and gas advection. The gas in the caprock of Field A shows decreasing molecular weight trend from deep to shallow depths implying migration from the underlying carbonate reservoir by gas diffusion. However, the gas in the caprock of Field B where there is a gas chimney visible in the seismic data, has composition similar to the gas in carbonate reservoir, suggesting that the gas came from carbonate reservoir below by gas advection through faults and induced fractures and occurred simultaneously with the gas accumulation in the reservoir. There is also gas in the caprock above the gas chimney with lighter molecular weight representing gas that migrated from the gas chimney by gas diffusion. The caprock seal capability in the two fields are different. The gas in the carbonate reservoir in Field A can be sealed and trapped by the high displacement/entry pressure of the capillary pore-throat system and the abnormally high pore pressure in the caprock. The gas chimney at Field B would be connected to the carbonate reservoir below over geological time and there is effective seal enough to contain hundreds ft of gas column in the carbonate reservoir. The understanding of the leaking mechanism in these two fields is helpful for understanding the leakage scale, the effects on the sealing capacity, the risk evaluation and mitigation amendment.

Gas hydrates identification and distribution in Mexican Ridges, Mexico

We mapped gas hydrates, free gas and Bottom Simulating Reflector (BSR) distributions in an area of Mexican Ridges, central Gulf of Mexico, Mexico, revealing the relationship between these three elements and the tectono-stratigraphy. The three elements are more visible when the host rock is a high porosity sandstone because there is a large seismic impedance contrast between solid gas hydrates above and free gas below, which manifests itself on the seismic as a BSR. Gas hydrates are identified in the well as higher resistivity sandstone layers with a strong positive amplitude. When the host rock is has a higher shale content with lower porosity, the impedance contrast is lower and the BSR is weak or not visible. On the other hand, Mexican Ridges are a series of anticlines where gas hydrates and free gas are trapped on the crest after migrates through the dipping layers and faults from synclines where are generated in calcareous shale. The main seal is MTC deposits from Pliocene, when they are not deposited at the crest of anticline there is gas escape o seafloor in form of gas chimney. On this way, we established a complete petroleum system for gas hydrates and free gas on Mexican Ridges.

DETERMINING 3D SEISMIC CHARACTERISTICS OF CONDUIT SYSTEM OF CHANGCHANG SAG, QIONGDONGNAN BASIN

Although some giant gas fields found in the deep-water area of the Qingdongnan Basin, China, are often associated with mud diapirs and/or gas chimneys, no comprehensive 3D work has been undertaken to characterize them. We conducted 3D seismic investigation using root mean squares (RMS), coherence, and instantaneous frequency attributes to provide better understanding of the conduit systems in the Qiongdongnan Basin. The results show that the conduit system that we investigated can be separated vertically into four zones in the following order. (1) A structurally diapiric weak zone at the base, followed by (2) an injected or reinjected sandstones zone, (3) a gas chimney zone, and (4) a mud volcanic zone at the top. The morphology of the structurally weak zone is elliptical, formed by the intersection of NW–SE– and nearly E–W–trending tectonic faults. We infer that this zone provides pathways for the ascent of the diapiric mud that was probably sourced by the underlying overpressured mudstones. The injected or reinjected sandstones zone is characterized by high amplitude anomalies (HAAs), and was probably fed by the lobes of underlying submarine fans. The gas chimney zone which is characterized by low frequencies and weak amplitudes, is probably composed of a mixture of uprising mud and free gas formed from the underlying overpressured mudstones; whereas, the mud volcano which has a Christmas-tree pattern, and composed of a central crater, the southern flank of which is a mudflow, formed when the uprising mud migrating upward through faults got to the paleo sea floor. Finally, we have proposed schematic illustrations that would aid in understanding the different stages of the formation and internal architecture of this conduit system.

Identifying gas chimneys in gas fields of the West Siberian petroleum basin via geophysical and drilling data

There are many push-down seismic anomalies (gas chimneys or pipes with a diameter of 12-15 km) in natural gas fields in the northern part of the West Siberian basin (Urengoy, Yamburg, Zapolarnoye, Kharasavey, Yurkharovskoye, etc.). This effect is common in gas-saturated sands. The amplitude of push-downs in West Siberian fields is approximately 300-400 ms from the horizontal surface. Using well data, we measure how much the velocity of seismic waves decreases and study the hydrocarbon presence, fluid pressure and other information to gain a better understanding of the cause of this effect. Several wells were drilled in three different gas chimneys. All the wells in the gas chimneys discovered vertical columnar of gas deposits in Jurassic and Cretaceous terrigenous formations. Well data from Yurkharovskoye gas field show that seismic wave velocity in the gas chimney is 40% less than that outside this object. The wells in the gas chimneys showed that formation pressure in the Neocomian and Jurassic formations is 80-90 MPa, which is approximately lithostatic pressure, so high-pressured gas in the pores begins to form cracks. There are gravity anomalies above many gas chimneys identified, indicating a density deficit. Transient electromagnetic sounding data from the Yurkharov gas chimney show a columnar zone of increased electrical resistance, as in gas-saturated formations. To explain all of these observations, we suppose that the West Siberian gas chimneys (pipes) are currently active channels for the vertical migration of high-pressured hydrocarbon gases (mainly methane), forming columnar gas-saturated zones. We think that the crucial cause of push-down seismic velocity anomalies is overpressure (approximately lithostatic). We predict that some new gas fields can be related to the gas chimneys at Gydan Peninsula and in Kara Sea. Many new deep gas deposits can be found in old gas fields with gas chimneys such as Urengoy and Yamburg.

Azimuthal anisotropy of Rayleigh waves across a gas chimney structure

<p>Gas chimneys are locations where natural gas leaks from the subsurface causing seabed pockmarks and potentially creating leakage pathways from CO<sub>2</sub> storage or other subsurface reservoirs. The CHIMNEY project seeks evidence of changes in anisotropy between a gas chimney and the surrounding sediments, which would corroborate theories on the chimney permeability being caused by fractures.</p><p>Twenty-five ocean bottom seismometers (OBSs) were placed in an asterisk-shaped array over the Scanner pockmark in the UK License Block 15/25 in the North Sea and at a reference location ~1.5 km away. The OBSs recorded for several days while an active source survey was undertaken. Rayleigh wave data were also available from ambient seismic noise observed by using a low pass filter to remove active sources from the data.</p><p>We use 2D beamforming to observe the azimuthal dependence of the Rayleigh wave phase velocity. 2D beamforming uses radon transforms summed over time for a range of different azimuths which gives the distribution of wave energy passing across an array as a function of group velocity.</p><p>Using narrowly band-passed data for the beamforming, we observe phase velocities of 250 - 650 m/s in the 0.8 - 1.2 Hz range. Initial results show θ, 2θ and 4θ anisotropy components in the measured phase velocities at the frequencies with the best ambient sources. We observe different fast orientations at the reference site than the chimney site. Varying anisotropy between the two sites supports the hypothesis that there is different fracturing in the chimney than in the surrounding geology.</p><p>With lower frequency surface waves penetrating deeper into the subsurface, dispersion of surface waves provides information about velocity variations with depth. Despite the array aperture imposing a lower limit on observable frequencies at around 0.7 Hz and noise source availability imposing a higher limit of about 1.2 Hz, strong dispersion was evident at both sites within this frequency window. The orientation and degree of anisotropy also appears to vary with frequency, indicating a variation in velocity and anisotropy with depth.</p><p>This work was undertaken with funding from NERC through the E3 Doctoral Training Partnership (E3 DTP; NE/L002558/1). The data was acquired with funding from the NERC (CHIMNEY; NE/N016130/1) and EU Horizon 2020 programme (STEMM-CCS; No.654462).</p>