In assessing methane hydrate as a potential transitional energy source, quantification of in-place volumes of gas is a first step—and yet, global, regional, and even local estimates of gas volumes contained within hydrate are highly variable, including within the extensively-studied Gulf of Mexico (GoM) gas hydrate province. Here, we construct the first 3-D basin and hydrocarbon system model of the Terrebonne Basin (Walker Ridge protraction area, northern GoM) to estimate in-place resources contained as gas within hydrate, as well as to provide the first estimate of in-place resources contained within free gas accumulations in the basin. The resultant model-predicted 3-D distributions (and saturations) of hydrate are consistent with independent seismic interpretation and borehole observations. This study reveals both sedimentation-driven and salt diapir-driven gas hydrate recycling produce elevated hydrate saturations in the basin (“recycling” occurs when relative upward shifts of the gas hydrate stability zone (GHSZ) cause hydrate dissociation, generating buoyant free gas that may reaccumulate as higher-saturation hydrate at the new base of GHSZ). An important finding of this study (particularly given that, globally, the source of methane in marine hydrate systems tends to be poorly understood) is that microbial sources can explain most or all hydrate occurrences in Terrebonne Basin. We calculate that ∼32 × 10⁹ m³ (∼1.1 10¹² ft³) of methane gas is trapped within hydrate accumulations throughout Terrebonne Basin, while nearly 2 × 10⁹ m³ (∼67 × 10⁹ ft³) of methane occurs as free gas at high saturations within sandy reservoirs beneath the GHSZ and ∼35 × 10⁹ m³ (∼1.2 × 10¹² ft³) occurs as free gas at low saturations within muddy units beneath the GHSZ. In total, our calibrated 3-D model predicts that nearly 70 × 10⁹ m³ (∼2.5 × 10¹² ft³) of microbial methane is trapped as gas hydrate and free gas in the Terrebonne Basin.