Methane Transport and Accumulation in Coarse-Grained Reservoirs in the Terrebonne Basin, Northern Gulf of Mexico

We studied methane migration mechanisms and associated hydrate accumulation rates in coarsegrained sands of the Terrebonne Basin, located in Walker Ridge Block 313, northern Gulf of Mexico. Hydrate in this area is distributed heterogeneously within ~900 m of methane hydrate stability zone, in both thick (10-25 m) and thin (< 3 m) sand layers, and in units of subvertical hydrate-filled fractures. We investigated hydrate formation from diffusively and advectively supplied methane using one-, two-, and three-dimensional basin modeling. We found that different migration mechanisms result in characteristic hydrate accumulation signatures that can link field observations to methane sources. Introduction Massive hydrate deposits, defined as thick accumulations of high hydrate saturation, have been encountered in many regions worldwide, including the Nankai Trough offshore Japan [1], the Krishna-Godavari Basin offshore India [2], the Ulleung Basin offshore Korea [3], and the outer continental shelf of the Gulf of Mexico [4]. As hydrates begin to be exploited as an energy resource, it is becoming more important to understand the mechanisms that lead to development of massive hydrate deposits in coarse-grained sediments, which are the most attractive reservoirs for exploration and production (e.g., [5]). Hydrates may be thought of broadly within a petroleum systems framework, requiring a methane source, migration mechanisms, a reservoir, and an appropriate seal. Hydrate reservoirs and seals are defined by thermodynamics rather than by buoyancy, as in the case of conventional oil and gas. Hydrates form most easily within coarse-grained sediments that occur in the methane hydrate stability zone (MHSZ), the depth interval in which pressure and temperature favor hydrate as the stable phase. Methane sources may include microbial degradation of organic matter as well as thermogenic processes [6]. We have focused on methane migration mechanisms as they represent some of the least understood processes in hydrate systems, but at the same time are a crucial link between sites of methane generation and hydrate reservoirs. Migration of methane into the MHSZ may occur in the dissolved phase or as a free gas phase. The methane flux and the phase of supplied methane determine the rate of hydrate formation, the amount of hydrate formed, and its distribution [7]. The accumulation and persistence of massive hydrate deposits therefore require a sufficient methane supply. We envision three migration mechanisms as illustrated in Figure 1: (A) diffusive flux within the MHSZ of dissolved methane from finer-grained units into coarser-grained units where hydrate forms preferentially, known as short migration [4,8,9]; (B) advective flux of dissolved methane from a source at depth into the MHSZ, known as long migration; or (C) migration of methane in the free gas phase, either produced locally within the MHSZ or supplied from the base of the MHSZ by long migration or from recycling of hydrate that is buried beneath the base of the MHSZ. The relative time scales and fluxes associated with these three migration mechanisms are largely unconstrained, but chiefly control the development and longevity of massive hydrate deposits that are attractive to energy resource development. To evaluate different migration mechanisms, we used a numerical simulations and data collected during the Chevron-led Gulf of Mexico Gas Hydrate Joint Industry Project (JIP) Leg 2 [10] at Walker Ridge Block 313 (WR313) in the Terrebonne Basin. Our work has demonstrated the importance of microbial methane, shortdistance migration, pore water advection, and lithologic heterogeneity on generating the massive hydrate deposits in the Terrebonne Basin. Figure 1: Illustration of methane (circles) migration mechanisms of dissolved methane (CH4) and methane bubbles (circles) associated with development and persistence of massive hydrate deposits (squares). Yellow layers represent coarser-grained units; brown represents finer-grained, clay-rich units. A: Short migration of methane from fine-grained layers into adjacent coarsergrained layers to form hydrate. B: Long migration of dissolved methane into the MHSZ. C: Methane migration in free gas phase within the MHSZ, either locally produced in fine-grained units or through fractures connected to the base of the MHSZ where gas may accumulate as hydrate-bearing units are buried, or updip from the base of the MHSZ within a coarse-grained unit. Hydrate distribution in the Terrebonne Basin Natural gas hydrates were first suspected in the northwestern Walker Ridge area by McConnell and Kendall [11]. They analyzed 3D seismic reflection data over the area and mapped an apparent base of the MHSZ by connecting the tops of high amplitude dim-outs in several tilted stratigraphic layers. These dim-outs, or phase-reversals, were thought to signal the transition from free gas to gas hydrate in permeable sand units. In 2009, JIP Leg 2 drilled two logging-while drilling (LWD) holes, Hole G and Hole H in WR313, on the edge of a salt withdrawal minibasin in 1950 m of water [10,12] (Fig. 2). These holes were located updip of some high-amplitude phase reversals visible on seismic data (Fig. 3). The base of the MHSZ at the WR313 site is about 900 mbsf (meters below seafloor). The high gas hydrate saturations confirmed the high-amplitude phase reversals on the seismic data were caused by a free gasto-gas hydrate transition [13]. Many more phase reversal regions have now been identified on seismic data in the Gulf of Mexico and are referred to as ‘partial BSRs’ (bottom simulating reflections [14,15]), suggesting many areas similar to WR313 site may exist. The target sand layer in WR313 Hole H was a deep sand layer up-dip of a clear phase reversal (Fig. 3), named the orange sand unit [16]. In Hole H, the orange sand unit occurs at from 806-819 mbsf and contains gas hydrate in 10100 cm sublayers with highly variable saturations ranging from 22-95% [17]. The target sand layer in WR313 Hole G, referred to as the blue sand unit (Fig. 3), occurs in Hole G from 774-878 mbsf and also contains a broad range of gas hydrate saturations in thin sublayers, though the blue sand has an overall lower net-to-gross ratio of sand to mud [16]. This same blue unit is again encountered up-dip in Hole H from 650-702 mbsf, and though the blue sand has thinned significantly it still contains gas hydrate. In addition to the main orange and blue units, several thin sands (less than 3 m) appear Figure 2: Location of the Terrebonne Basin in the Gulf of Mexico. Figure 3: A seismic cross section at WR313 through the two JIP Leg 2 holes, Hole G and Hole H as well as a previously drilled industry well. The gamma ray log (GR), a lithology indicator, is shown in green at each hole. A propagation resistivity log (RES) is shown in maroon. The main sands modeled in this study are identified: the red sand, the blue sand and the orange sand units. A clear phase reversal can be seen near Hole H, slightly down-dip from the orange sand unit. A strata-bound, clay-rich interval containing fracture-hosted hydrate is shaded in green. throughout both wells and some (though not all) contain natural gas hydrate [16]. Another interesting gas hydrate accumulation at WR313 occurs in a strata-bound, clay-rich interval that contains gas hydrate in near-vertical fractures. The strata-bound interval was encountered in Hole G (235-403 mbsf), Hole H (160-316 mbsf) and the updip industry well (146-277 mbsf) (green shaded area in Fig. 3). A thin, hydrate-bearing sand, called the red sand (or unit A in Cook and Malinverno [8]), occurs near the base of the strata-bound unit. The red sand is about 2.5 m thick and is surrounded by several meters of hydrate-free clay. While the seismic identification technique and drilling program at Walker Ridge was clearly successful, it remains unclear how the sand layers at Walker Ridge became saturated with natural gas hydrate. The large hydrate saturations are vertically heterogeneous but present through nearly 1 km of sediment within the MHSZ. On the other hand, the large saturations exhibit relatively limited lateral extent, with continuity of perhaps 1 km in a dipping coarse-grained layer (elevated resistivities in any given layer typically occur in one or two but rarely all three wells in Fig. 3). Because of the bright spots observed directly down dip of the base of the MHSZ the gas hydrate saturated sand layers, Boswell et al. [4] and Frye et al. [16] conclude that the most likely explanation is lateral migration of natural gas along the sand unit from a deeper source, or “long migration”. In contrast, Cook and Malinverno [8] argue that hydrate in the red sand could have been supplied by short migration. In addition, Cook and Malinverno [8] note that the presence of free gas below a gas hydrate saturated unit does not necessarily imply that the methane was derived from below, as the free gas below the MHSZ may be the result of dissociated gas hydrate that could have been formed from microbial methane within the MHSZ and eventually buried below the base of the MHSZ.